Fireproof
Astounding Science Fiction, March 1949
(1949)*
Hal Clement

	This yarn, gentlemen, introduces a brand new idea in the field of a spaceship operation. There's twenty years of discussion gone by—and this beautiful, simple, and exceedingly neat point has been totally missed! Before you reach the end, see if you can figure out the answer!

-

	Hart waited a full hour after the last sounds had died away before cautiously opening the cover of his refuge. Even then he did not feel secure for some minutes, until he had made a thorough search of the storage chamber; then a smile of contempt curled his lips. 

	"The fools!" he muttered. "They do not examine their shipments at all. How do they expect to maintain their zone controls with such incompetents in charge?" He glanced at the analyzers in the forearm of his spacesuit, and revised his opinion a trifle—the air in the chamber was pure carbon dioxide; any man attempting to come as Hart had, but without his own air supply, would not have survived the experiment. Still, the agent felt, they should have searched. 

	There was, however, no real time for analyzing the actions of others. He had a job to do, and not too long in which to do it. However slack the organization of this launching station might be, there was no chance whatever of reaching any of its vital parts unchallenged; and after the first challenge, success and death would be running a frightfully close race. 

	He glided back to the crate which had barely contained his doubled-up body, carefully replaced and resealed the cover, and then rearranged the contents of the chamber to minimize the chance of that crate's being opened first. The containers were bulky, but nothing in the free-falling station had any weight, and the job-did not take long even for a man unaccustomed to a total lack of apparent gravity. Satisfied with these precautions, Hart approached the door of the storeroom; but before opening it, he stopped to review his plan. 

	He must, of course, be near the outer shell of the station. Central Intelligence had been unable to obtain plans of this launcher—a fact which should have given him food for thought—but there was no doubt about its general design. Storage and living quarters would be just inside the surface of the sphere; then would come a level of machine shops and control systems; and at the heart, within the shielding that represented most of the station's mass, would be the "hot" section—the chambers containing the fission piles and power plants, the extractors and the remote-controlled machinery that loaded the war heads of the torpedoes which were the main reason for the station's existence. 

	There were many of these structures circling Earth; every nation on the globe maintained at least one, and usually several. Hart had visited one of those belonging to his own country, partly for technical familiarity and partly to accustom himself to weightlessness. He had studied its plans with care, and scientists had carefully explained to him the functions of each part, and the ways in which the launchers of the Western Alliance were likely to differ. Most important, they had described to him several ways by which such structures might be destroyed. Hart's smile was wolfish as he thought of that; these people who preferred the pleasures of personal liberty to those of efficiency would see what efficiency could do. 

	But this delay was not efficient. He had made his plans long before, and it was more than time to set about their execution. He must be reasonably near a store of rocket fuel; and some at least of the air in this station must contain a breathable percentage of oxygen. Without further deliberation, he opened the door and floated out into the corridor. 

	He did not go blindly. Tiny detectors built into the wrists of his suit reacted to the infrared radiations, the water vapor and carbon dioxide and even the breathing sounds that would herald the approach of a human being—unless he were wearing a nonmetallic suit similar to Hart's own. Apparently the personnel of the base did not normally wear these, however, for twice in the first ten minutes the saboteur was warned into shelter by the indications of the tiny instruments. In that ten minutes he covered a good deal of the outer zone. 

	He learned quickly that the area in which a carbon dioxide atmosphere was maintained was quite limited in extent, and probably constituted either a quarantine zone for newly arrived supplies, or a food storage area. It was surrounded by an uninterrupted corridor lined on one side with airtight doors leading into the COs rooms, and on the other by flimsier portals closing off other storage spaces. Hart wondered briefly at the reason for such a vast amount of storage room; then his attention was taken by another matter. He had been about to launch himself in another long, weightless glide down the corridor in search of branch passages which might lead to the rocket fuel stores, when a tiny spot on one wall caught his eye. 

	He instantly went to examine it more closely, and as quickly recognized a photoelectric eye. There appeared to be no lens, which suggested a beam-interruption unit; but the beam itself was not visible, nor could he find any projector. That meant a rather interesting and vital problem lay in avoiding the ray. He stopped to think. 

-

	In the scanning room on the second level, Dr. Bruce Mayhew chuckled aloud. 

	"It's wonderful what a superiority complex can do. He's stopped for the first time—didn't seem to have any doubts of his safety until he spotted that eye. The old oil about 'decadent democracies' seems to have taken deep hold somewhere, at least. He must be a military agent rather than a scientist." 

	Warren Floyd nodded. "Let's not pull the same boner, though," he suggested. "Scientist or not, no stupid man would have been chosen for such a job. Do you think he's carrying explosives? One man could hardly have chemicals enough to make a significant number of breaches in the outer shell." 

	"He may be hoping to get into the core, to set off a war head," replied the older man, "though I don't for the life of me see how he expects to do it. There's a rocket fuel in his neighborhood, of course, but it's just n.v. for the torpedoes—harmless, as far as we're concerned." 

	"A fire could be quite embarrassing, even if it weren't an explosion," pointed out his assistant, "particularly since the whole joint is nearly pure magnesium. I know it's sinfully expensive to transport mass away from Earth, but I wish they had built this place out of something a little less responsive to heat and oxygen." 

	"I shouldn't worry about that," replied Mayhew. "He won't get a fire started." 

	Floyd glanced at the flanking screens which showed armored men keeping pace with the agent in parallel corridors, and nodded. "I suppose not—provided Ben and his crew aren't too slow closing in when we give the signal." 

	"You mean when I give the signal," returned the other man. "I have reasons for wanting him free as long as possible. The longer he's free, the lower the opinion he'll have of us; when we do take him, he'll be less ready to commit suicide, and the sudden letdown of his self-confidence will make interrogation easier." 

	Floyd privately hoped nothing would happen to deflate his superior's own self-confidence, but wisely said nothing; and both men watched Hart's progress almost silently for some minutes. Floyd occasionally transmitted a word or two to the action party to keep them apprised of their quarry's whereabouts, but no other sound interrupted the vigil. 

-

	Hart had finally found a corridor which branched away from the one he had been following, and he proceeded cautiously along it. He had learned the intervals at which the photocells were spotted, and now avoided them almost automatically. It did not occur to him that, while the sight of a spacesuited man in the outer corridors might not surprise an observer, the presence of such a man who failed consistently to break the beams of the photocell spotters would be bound to attract attention. The lenses of the scanners were too small and too well hidden for Hart to find easily, and he actually believed that the photocells were the only traps. With his continued ease in avoiding them, his self-confidence and contempt for the Westerners were mounting as Mayhew had foretold. 

	Several times he encountered air breaks—sliding bulkheads actuated by automatic pressure-controlled switches, designed to cut off any section with a bad air leak. His action at each of these was the same; from an outer pocket of his armor he would take a small wedge of steel and skillfully jam the door. It was this action which convinced Mayhew that the agent was not a scientist—he was displaying the skill of an experienced burglar or spy. He was apparently well supplied with the wedges, for in the hour before he found what he was seeking he jammed more than twenty of the air breaks. Mayhew and Floyd did not bother to have them cleared at the time, since no one was in the outer level without a spacesuit. 

	Nearly half of the outer level was thus unified when Hart reached a section of corridor bearing valve handles and hose connections instead of doors, and knew there must be liquids behind the walls. There were code indexes stenciled over the valves, which meant nothing to the spy; but he carefully manipulated one of the two handles to let a little fluid into the corridor, and sniffed at it cautiously through the gingerly cracked face plate of his helmet. He was satisfied with the results; the liquid was one of the low-volatility hydrocarbons used with liquid oxygen as a fuel to provide the moderate acceleration demanded by space launched torpedoes. They were cheap, fairly dense, and their low-vapor pressure simplified the storage problem in open-space stations. 

	All that Hart really knew about it was that the stuff would burn as long as there was oxygen. Well—he grinned again at the thought—there would be oxygen for a while; until the compressed, blazing combustion gases blew the heat-softened metal of the outer wall into space. After that there would be none, except perhaps in the central core, where the heavy concentration of radioactive matter made it certain there would be no one to breathe it. 

	At present, of course, the second level and any other intermediate ones were still sealed; but that could and would be remedied. In any case, the blast of the liberated fuel would probably take care of the relatively flimsy inner walls. He did not at the time realize that these were of magnesium, or he would have felt even more sure of the results. 

	He looked along the corridor. As far as the curvature of the outer shell permitted him to see, the valves projected from the wall at intervals of a few yards. Each valve had a small electric pump, designed to force air into the tank behind it to drive the liquid out by pressure, since there was no gravity. Hart did not consider this point at all; a brief test showed him that the liquid did flow when the valve was on, and that was enough for him. Hanging poised beside the first handle, he took an object from still another pocket of his spacesuit, and checked it carefully, finally clipping it to an outside belt where it could easily be reached. 

-

	At the sight of this item of apparatus, Floyd almost suffered a stroke. 

	"That's an incendiary bomb!" he gasped aloud. "We can't possibly take him in time to stop his setting it off which he'll do the instant he sees our men! And he already has free fuel in the corridor!" 

	He was perfectly correct; the agent was proceeding from valve to valve in long glides, pausing at each just long enough to turn it full on and to scatter the balloon-like mass of escaping liquid with a sweep of his arm. Gobbets and droplets of the inflammable stuff sailed lazily hither and yon through the air in his wake. 

	Mayhew calmly lighted a cigarette, unmindful of the weird appearance of the match flame driven toward his feet by the draft from the ceiling ventilators, and declined to move otherwise. "Decidedly, no physicist," he murmured. "I suppose that's just as well—it's the military information the army likes anyway. They certainly wouldn't have risked a researcher on this sort of job, so I never really did have a chance to get anything I wanted from him." 

	"But what are we going to do?" Floyd was almost frantic. "There's enough available energy loose in that corridor now to blast the whole outer shell off—and gallons more coming every second. I know you've been here a lot longer than I, but unless you can tell me how you expect to keep him from lighting that stuff up, I'm getting into a suit right now!" 

	"If it blows, a suit won't help you," pointed out the older man. 

	"I know that!" almost screamed Floyd, "but what other chance is there? Why did you let him get so far?" 

	"There is still no danger," Mayhew said flatly, "whether you believe it or not. However, the fuel does cost money, and there'll be some work recovering it, so I don't see why he should be allowed to empty all the torpedo tanks. He's excited enough now, anyway." He turned languidly to the appropriate microphone and gave the word to the action squad. "Take him now. He seems to be without hand weapons, but don't count on it. He certainly has at least one incendiary bomb." As an afterthought, he reached for another switch, and made sure the ventilators in the outer level were not operating; then he relaxed again and gave his attention to the scanner that showed the agent's activity. Floyd had switched to another pickup that covered a longer section of corridor, and the watchers saw the space-suited attackers almost as soon as did Hart himself. 

-

	The European reacted to the sight at once—too rapidly, in fact, for the shift in his attention caused him to miss his grasp on the valve handle he sought and flounder helplessly through the air until he reached the next. Once anchored, however, he acted as he had planned, ignoring with commendable self-control the four armored figures converging on him. A sharp twist turned the fuel valve full on, sending a stream of oil mushrooming into the corridor; his left hand flashed to his belt, seized the tiny cylinder he had snapped there, jammed its end hard against the adjacent wall, and tossed the bomb gently back down the corridor. In one way his lack of weightless experience betrayed him; he allowed for a gravity pull that was not there. The bomb, in consequence, struck the "ceiling" a few yards from his hand, and rebounded with a popping noise and a shower of sparks. It drifted on down the corridor toward the floating globules of hydrocarbon, and the glow of the sparks was suddenly replaced by the eye-hurting radiance of thermite. 

	Floyd winced at the sight, and expected the attacking men to make futile plunges after the blazing thing; but though all were within reach of walls, not one swerved from his course. Hart made no effort to escape or fight; he watched the course of the drifting bomb with satisfaction, and, like Floyd, expected in the next few seconds to be engulfed in a sea of flame that would remove the most powerful of the Western torpedo stations from his country's path of conquest. Unlike Floyd, he was calm about it, even when the men seized him firmly and began removing equipment from his pockets. One unclamped and removed the face plate of his helmet; and even to that he made no resistance—just watched in triumph as his missile drifted toward the nearest globes of fuel. 

	It did not actually strike the first. It did not have to; while the quantity of heat radiated by burning thermite is relatively small, the temperature of the reaction is notoriously high—and the temperature six inches from the bomb was well above the flash point of the rocket fuel, comparatively non-volatile as it was. Floyd saw the flash as its surface ignited, and closed his eyes. 

	Mayhew gave him four or five seconds before speaking, judging that that was probably about all the suspense the younger man could stand. 

	"All right, ostrich," he finally said quietly. "I'm not an angel, in case you were wondering. Why not use your eyes, and the brain behind them?" 

	Floyd was far too disturbed to take offense at the last remark, but he did cautiously follow Mayhew's advice about looking. He found difficulty, however, in believing what his eyes and the scanner showed him. 

	The group of five men was unchanged, except for the expression on the captive's now visible face. All were looking down the corridor toward the point where the bomb was still burning; Lang's crew bore expressions of amusement on their faces, while Hart wore a look of utter disbelief. Floyd, seeing what he saw, shared the expression. 

	The bomb had by now passed close to several of the floating spheres. Each had caught fire, as Floyd had seen—for a moment only. Now each was surrounded by a spherical, nearly opaque layer of some grayish substance that looked like a mixture of smoke and kerosene vapor; a layer that could not have been half an inch thick, as Floyd recalled the sizes of the original spheres. None was burning; each had effectively smothered itself out, and the young observer slowly realized just how and why as the bomb at last made a direct hit on a drop of fuel fully a foot in diameter. 

	Like the others, the globe flamed momentarily, and went out; but this time the sphere that appeared and grew around it was lighter in color, and continued to grow for several seconds. Then there was a little, sputtering explosion, and a number of fragments of still burning thermite emerged from the surface of the sphere in several directions, traveled a few feet and went out. All activity died down, except in the faces of Hart and Floyd. 

-

	The saboteur was utterly at a loss, and seemed likely to remain that way; but in the watch room Floyd was already kicking himself mentally for his needless worry. Mayhew, watching the expression on his assistant's face, chuckled quietly. 

	"Of course you get it now," he said at last. 

	"I do now, certainly," replied Floyd. "I should have seen it earlier—I've certainly noticed you light enough cigarettes, and watched the behavior of the match flame. Apparently our friend is not yet enlightened, though," he nodded toward the screen as he spoke. 

	He was right; Hart was certainly not enlightened. He belonged to a service in which unpleasant surprises were neither unexpected nor unusual, but he had never in his life been so completely disorganized. The stuff looked like fuel; it smelled like fuel; it had even started to burn—but it refused to carry on with the process. Hart simply relaxed in the grip of the guards, and tried to find something in the r, situation to serve as an anchor for his whirling thoughts. A spaceman would have understood the situation without thinking, a high school student of reasonable intelligence could probably have worked the matter out in time; but Hart's education had been that of a spy, in a country which considered general education a waste of time. He simply did not have the background to cope with his present environment. 

	That, at least, was the idea Mayhew acquired after a careful questioning of the prisoner. Not much was learned about his intended mission, though there was little doubt about it under the circumstances. The presence of an alien agent aboard any of the free-floating torpedo launchers of the various national governments bore only one interpretation; and since the destruction of one such station would do little good to anyone, Mayhew at once radioed all other launchers to be on the alert for similar intruders—all others, regardless of nationality. Knowledge by Hart's superiors of his capture might prevent their acting on the assumption that he had succeeded, which would inevitably lead to some highly regrettable incidents. Mayhew's business was to prevent a war, not win one. Hart had not actually admitted the identity of his superiors, but his accent left the matter in little doubt; and since no action was intended, Mayhew did not need proof. 

	There remained, of course, the problem of what to do with Hart. The structure had no ready-made prison, and it was unlikely that the Western government would indulge in the gesture of a special rocket to take the man off. Personal watch would be tedious, but it was unthinkable merely to deprive a man with the training Hart must have received of his equipment, and then assume he would not have to be watched every second. 

	The solution, finally suggested by one of the guards, was a small storeroom in the outer shell. It had no locks, but there were welding torches in the machine shops. There was no ventilator either, but an alga tank would take care of that. After consideration, Mayhew decided that this was the best plan, and it was promptly put into effect. 

-

	Hart was thoroughly searched, even his clothing replaced as a precautionary measure. He asked for his cigarettes and lighter, with a half smile. Mayhew supplied the man with some of his own, and marked those of the spy for special investigation. Hart said nothing more after that, and was incarcerated without further ceremony. Mayhew was chuckling once more as the guards disappeared with their charge. 

	"I hope he gets more good than I out of that lighter," he remarked. "It's a wick-type my kid sent me as a present, and the ventilator draft doesn't usually keep it going. Maybe our friend will learn something, if he fools with it long enough. He has a pint of lighter fluid to experiment with—the kid had large ideas." 

	"I was a little surprised—I thought for a moment you were giving him a pocket flask," laughed Floyd. "I suppose that's why you always use matches—they're easier to wave than that thing. I guess I save myself a lot of trouble not smoking at all. I suppose you have to put potassium nitrate in your cigarettes to keep 'em going when you're not pulling on them." Floyd ducked as he spoke, but Mayhew didn't throw anything. Hart, of course, was out of hearing by this time, and would not have profited from the remark in any case. 

	He probably, in fact, would not have paid much attention. He knew, of course, that the sciences of physics and chemistry are important; but he thought of them in connection with great laboratories and factories. The idea that knowledge of either could be of immediate use to anyone not a chemist or physicist would have been fantastic to him. While his current plans for escape were based largely on chemistry, the connection did not occur to him. The only link between those plans and Mayhew's words or actions gave the spy some grim amusement; it was the fact that he did not smoke. 

	The cell, when he finally reached it, was perfectly satisfactory; there were no peepholes which could serve as shotholes, no way in which the door could be unsealed quickly—as Mayhew had said, not even a ventilator. Once he was in, Hart would not be interrupted without plenty of notice. Since the place was a storeroom, there was no reason to expect even a scanner, though, he told himself, there was no reason to assume there was none, either. He simply disregarded that possibility, and went to work the moment he heard the torch start to seal his door. 

	His first idea did not get far. He spent half an hour trying to make Mayhew's lighter work, without noticeable success. Each spin of the "flint" brought a satisfactory shower of sparks, and about every fourth or fifth try produced a faint "pop" and a flash of blue fire; but he was completely unable to make a flame last. He closed the cover at last, and for the first time made an honest effort to think. The situation had got beyond the scope of his training. 

	He dismissed almost at once the matter of the rocket fuel that had not been ignited by his bomb. Evidently the Westerners stored it with some inhibiting chemical, probably as a precaution more against accident than sabotage. Such a chemical would have to be easily removable, but he had no means of knowing the method, and that line of attack would have to be abandoned. 

	But why wouldn't the lighter fuel burn? The more he thought the matter out, the more Hart felt that Mayhew must have doctored it deliberately, as a gesture of contempt. Such an act he could easily understand; and the thought of it roused again the wolfish hate that was such a prominent part of his personality. He would show that smart Westerner! There was certainly some way! 

	Powerful hands, and a fingernail deliberately hardened long since to act as a passable screw-driver blade, had the lighter disassembled in the space of a few minutes. The parts were disappointingly small in number and variety; but Hart considered each at length. 

	The fuel, already evaporating as it was, appeared useless—he was no chemist, and had satisfied himself the stuff was incombustible. The case was of magnalium, apparently; and might be useful as a heat source if it could be lighted; its use in a cigarette lighter did not encourage pursuit of that thought. The wick might be combustible, if thoroughly dried. The flint and wheel mechanism was promising—at least one part would be hard enough to cut or wear most metals, and the spring might be decidedly useful. 

	Elsewhere in the room there was very little. The light was a gas tube, and, since the chamber had no opening whatever, would probably be most useful as a light. The alga tank, of course, had a minute motor and pump which forced air through its liquid, and an ingenious valve and trap system which recovered the air even in the present weightless situation; but Hart, considering the small size of the room, decided that any attempt to dismantle his only source of fresh air would have to be very much of a last resort. 

-

	After much thought, and with a grimace of distaste, he took the tiny striker of the lighter and began slowly to abrade a circular area around the latch of the door, using the inside handle for anchorage. 

	He did not, of course, have any expectation of final escape; he was not in the least worried about his chances of recovering his spacesuit. He expected only to get out of the cell and complete his mission; and if he succeeded, no possible armor would do him any good. 

	As it happened, there was a scanner in his compartment; but Mayhew had long since grown tired of watching the spy try to ignite the lighter fuel, and had turned his attention elsewhere, so that Hart's actions were unobserved for some time. The door metal was thin and not particularly hard; and he was able without interference and with no worse trouble than severe finger cramp to work out a hole large enough to show him another obstacle—instead of welding the door frame itself, his captors had placed a rectangular steel bar across the portal and fastened it at points well to each side of the frame, out of the prisoner's reach. Hart stopped scraping as soon as he realized the extent of this barrier, and gave his mind to the new situation. 

	He might, conceivably, work a large enough hole through the door to pass his body without actually opening the portal; but his fingers were already stiff and cramped from the use made of the tiny striker, and it was beyond reason to expect that he would be left alone long enough to accomplish any such feat. Presumably they intended to feed him occasionally. 

	There was another reason for haste, as well, though he was forgetting it as his nose became accustomed to the taint in the air. The fluid, which he had permitted to escape while disassembling the lighter, was evaporating with fair speed, as it was far more volatile than the rocket fuel; and it was diffusing through the air of the little room. The alga tank removed only carbon dioxide, so that the air of the cell was acquiring an ever-greater concentration of hydrocarbon molecules. Prolonged breathing of such vapors is far from healthy, as Hart well knew; and escape from the room was literally the only way to avoid breathing the stuff. 

	What would eliminate a metal door—quickly? Brute force? He hadn't enough of it. Chemicals? He had none. Heat? The thought was intriguing and discouraging at the same time, after his recent experience with heat sources. Still, even if liquid fuels would not burn perhaps other things would: there was the wicking from the lighter; a little floating cloud of metal particles around the scene of his work on the magnesium door; and the striking mechanism of the lighter. 

	He plucked the wicking out of the air where it had been floating, and began to unravel it—without fuel, as he realized, it would need every advantage in catching the sparks of the striker. 

	Then he wadded as much of the metallic dust as he could collect—which was not too much—into the wick, concentrating it heavily at one end and letting it thin out toward the more completely raveled part. 

	Then he inspected the edges of the hole he had ground in the door, and with the striker roughened them even more on one side, so that a few more shavings of metal projected. To these he pressed the fuse, wedging it between the door and the steel bar just outside the hole, with the "lighting" end projecting into the room. He inspected the work carefully, nodded in satisfaction, and began to reassemble the striker mechanism. 

	He did not, of course, expect that the steel bar would be melted or seriously weakened by an ounce or so of magnesium, but he did hope that the thin metal of the door itself would ignite. 

-

	Hart had the spark mechanism almost ready when his attention was distracted abruptly. Since the hole had been made, a very gentle current of air had been set up in the cell by the corridor ventilators beyond—a current in the nature of an eddy which tended to carry loose objects quite close to the hole. One of the loose objects in the room was a sphere comprised of the remaining lighter fluid, which had not yet evaporated. When Hart noticed the shimmering globe, it was scarcely a foot from his fuse, and drifting steadily nearer. 

	To him, that sphere of liquid was death to his plan; it would not burn itself, it probably would not let anything else burn either. If it touched and soaked his fuse, he would have to wait until it evaporated; and there might not be time for that. He released the striker with a curse, and swung his open hand at the drop, trying to drive it to one side. He succeeded only partly. It spattered on his hand, breaking up into scores of smaller drops, some of which moved obediently away, while others just drifted, and still others vanished in vapor. None drifted far; and the gentle current had them in control almost at once, and began to bear many of them back toward the hole—and Hart's fuse. 

	For just a moment the saboteur hung there in agonized indecision, and then his training reasserted itself. With another curse he snatched at the striker, made sure it was ready for action, and turned to the hole in the door. It was at this moment that Mayhew chose to take another look at his captive. 

	As it happened, the lens of his scanner was so located that Hart's body covered the hole in the door; and since the spy's back was toward him, the watcher could not tell precisely what he was doing. The air of purposefulness about the captive was so outstanding and so impressive, however, that Mayhew was reaching for a microphone to order a direct check on the cell when Hart spun the striker wheel. 

	Mayhew could not, of course, see just what the man had done, but the consequences were plain enough. The saboteur's body was flung away from the door and toward the scanner lens like a rag doll kicked by a mule. An orange blossom of flame outlined him for an instant; and in practically the same instant the screen went blank as a heavy shock wave shattered its pickup lens. 

	Mayhew, accustomed as he was to weightless maneuvering, never in his life traveled so rapidly as he did then. Floyd and several other crewmen, who saw him on the way, tried to follow; but he outstripped them all, and when they reached the sight of Hart's prison Mayhew was hanging poised outside, staring at the door. 

	There was no need of removing the welded bar. The thin metal of the door had been split and curled outward fantastically; an opening quite large enough for any man's body yawned in it, though there was nothing more certain than the fact that Hart had not made use of this avenue of escape. His body was still in the cell, against the far wall; and even now the relatively strong currents from the hall ventilators did not move it. Floyd had a pretty good idea of what held it there, and did not care to look closely. He might be right. 

-

	Mayhew's voice broke the prolonged silence. 

	"He never did figure it out." 

	"Just what let go, anyway?" asked Floyd. 

	"Well, the only combustible we know of in the cell was the lighter fluid. To blast like that, though, it must have been almost completely vaporized, and mixed with just the right amount of air—possible, I suppose, in a room like this. I don't understand why he let it all out, though." 

	"He seems to have been using pieces of the lighter," Floyd pointed out. "The loose fuel was probably just a by-product of his activities. He was even duller than I, though. It took me long enough to realize that a fire needs air to burn—and can't set up convection currents to keep itself supplied with oxygen, when there is no gravity." 

	"More accurately, when there is no weight," interjected Mayhew. "We are well within Earth's gravity field, but in free fall. Convection currents occur because the heated gas is lighter per unit volume than the rest, and rises. With no weight, and no 'up' such currents are impossible." 

	"In any case, he must have decided we were fooling him with noncombustible liquids." 

	Mayhew replied slowly: "People are born and brought up in a steady gravity field, and come to take all its manifestations for granted. It's extremely hard to foresee all the consequences which will arise when you dispense with it. I've been here for years, practically constantly, and still get caught sometimes when I'm tired or just waking up." 

	"They should have sent a spaceman to do this fellow's job, I should think." 

	"How would he have entered the station? A man is either a spy or a spaceman—to be both would mean he was too old for action at all, I should say. Both professions demand years of rigorous training, since habits rather than knowledge are required—habits like the one of always stopping within reach of a wall or other massive object." There was a suspicion of the old chuckle in his voice as Mayhew spoke the final sentence, and it was followed by a roar of laughter from the other men. Floyd looked around, and blushed furiously. 

	He was, as he had suspected from the older man's humor, suspended helplessly in midair out of reach of every source of traction. Had there been anything solid around, he would probably have used it for concealment instead, anyway. He managed at last to join that laughter; but at its end he glanced once more into Hart's cell, and remarked, "If this is the worst danger that inexperience lands on my head, I don't think I'll complain. Bruce, I want to go with you on your next leave to Earth; I simply must see you in a gravity field. I bet you won't wait for the ladder when we step off the rocket—though I guess it would be more fun to see you drop a dictionary on your toe. As you implied, habits are hard to break." 

The End