plump doctor stood beside the door saluting; as his hand flew up, Von Marx raised himself on his pillows. The moment was at hand and he was ready for it.

The general came in, a spare man, with a gray, mobile face like a clever ape, and alert, mocking eyes under heavy brows. Behind him, his big officers crowded the little, low-ceilinged bedroom, making a foil of mere largeness and physical strength to his pungent and forcible personality. He cocked a swift, searching eye at the young man on the bed; it somehow expressed, in the same glance, both indulgence and severity.

“Ah, Lieutenant!" he barked, in his shrill, grating voice. "Getting better, aren't you? Of course you are. Fine thing you did the other night. Very fine thing!"

There was malice in the manner in which he said it; if he had wished to jeer, to insult, he could not have spoken it otherwise. Von Marx glanced toward the staff officers, wondering for a moment if he had been forestalled and the whole shameful thing discovered without his help. But they knew nothing: for them, General Kraft was speaking in his ordinary tones; he was famous for these and for the rasp of his

manner.

"General," began Von Marx.

66

"Hey!" interrupted the general. "You oughtn't to talk, you know." He jerked his impish old face round toward the doctor. Bad for him to talk, isn't it? Of course it is. There, you see," to Von Marx again; "the worst thing you can do is to talk. It might-oh, it might get you into all sorts of trouble."

It was unmistakable-he knew! Von Marx, gazing at him helplessly, felt a horror of his cunning and his glee. He had found it all out ad was relishing it, chewing the cud of it and finding it

"It has been easy," began the general, "to read the story of your deed, and what was lacking in the evidence was supplied by the accounts of the survivors and the prisoners." His gaze was a warning now; the malice had gone out of it. As clearly as though he had spoken in words, it told Von Marx that he had a part to play, a rôle to fulfil, by keeping silent.

Von Marx lay still, watching him, bewildered and daunted. The general gave him a slow glare and half turned toward his staff officers so as to include them in his audience. He began to speak deliberately, pausing to choose words, so that his little speech gave to the affair the air of a ceremony. The tall, uniformed men, spruce and comely, drew themselves up formally to hear him; he made of the little, plain chamber a council-room.

Gentlemen," he said, a man Owes to his country what he has the brave their courage, the strong their strength— and those who have most owe the largest debt. The most heavily indebted of all are those who owe to their fatherland not merely service in the field, but service, devotion, sacrifice, at all hours, while life remains in them. Such men are few; we know them first by some such deed as this which Lieutenant von Marx has accomplished; we distinguish them thereafter by-this token!"

His restless fingers came away from his breast and he held up before them, in his bony, red hand, the little fivepointed star of dull iron, dangling by its black ribbon. There was an instant of silence. Then he went about the corner of the bed.

"Take it," he said to Von Marx. "Take it. You must pin it on yourself; it is the rule. Take it, I tell you."

Under the compulsion of his eyes and voice, Von Marx received it in his open hand. He was aghast; the thing he had intended to prevent was happening in spite of him.

"I must speak," he cried, suddenly. "I will speak. This star-I can't-"

The general interrupted him in his harshest voice. He looked across at his officers.

"Then," he said, "in that case you will wait for me below, gentlemen."

"Well," he said. "You want to tell me what you were doing when the strawyard was fired, eh? Isn't that it? By the way "-he cocked his bright and secret eye inquisitively," who did set the straw on fire?"

Von Marx gaped helplessly.

"Schmidt, I suppose?" went on the general. "Poor Schmidt! I saw him

shot in our first action. A mere bulldog of a man; no soldier at all. However, he's been some use at last."

Then," cried Von Marx, "you know! You know everything?"

The general nodded. "I know everything," he said, steadily.

Ah, but you don't-you can't," cried the younger man. He flung himself up on his elbow.

"It was in an orchard

that it began," he said, his words tumbling over one another in his eagerness to tell the whole miserable story and fulfil his purpose. "Schmidt had lingered behind to light a cigarette. One of the enemy-with a revolver-" He paused.

"I know, I know," said the general. "That man wasn't dead when we got here, and I spoke to him before he died. I know it all."

"But "-Von Marx touched the Iron Star where it lay on the counterpane"but this! If you know all-" stopped.

He

The general frowned thoughtfully, put his hands behind him, and walked away to the foot of the bed.

"Ah," he said, "I must make you

To be brave alone is not enough to win it; we have brave men in plenty. And to be a great soldier is not enough. It goes to those who have done more than our country can ask of its sons-men who are to heroes what apostles are to priests. To win it a man must be inhuman-and there are no such men. It never has been won; it never will be." His eyes were empty now of mockery; he was grave, simple, almost reverent.

"It carries an obligation," he went on; "the obligation to serve whole-heartedly, with every faculty of mind and body, till you have expiated your dishonor. You will never be able to rest till you have done it; the star won't let you. Each time you receive the salute of honor you will feel it sting you like a whip. You can never be a coward again; you will not dare. Pin it on your breast, Von Marx; that is the first step, and nobody can do it for you. Pin it on!"

"Ah!" The sound was almost a sob. Fumbling with nerveless fingers, Von Marx found the star and fastened it on his shirt over his heart. The pin, as he thrust it through, pricked him sharply.

The general came again to his bedside.

"Welcome," he said, holding out his hand. "Welcome to the order. There were thirty-two of us; you make the thirty-third. Thirty-two men will know the truth about what you have done, therefore. They will understand."

"Will they?" asked Von Marx.

The general nodded again. "Yes," he said. "For each of them gained the star in the same way. I, too-yes; I, too! There is no other way."

His hand was still outstretched, in comradeship and kindness. Von Marx took it, wondering.

I'

we see, and indeed many things which we do not see, such as air, are composed of atoms. Till within a few years it was believed on indirect evidence that this was so; now we know that it is.

What is an atom? The meaning of the word is "uncuttable," "indivisible." For the origin of the conception we must go back to the times of the ancients. What we know of ancient literature is confined to the writings of the Assyrians, the Egyptians, the inhabitants of India, of Greece, and of Rome. But their writings treat of history or of poetry, as a rule; the only scientific pursuits of the inhabitants of these countries were politics, ethics, and mathematics. Distinction was to be gained in the forum, in the temple, or on the battle field, not in wresting secrets from nature.

Some of the ancient Greek authors, however, speculated on the world which they inhabited; among them, Democritus, Empedocles, Epicurus, and Aristotle. One of the questions which they discussed was whether the matter of which they believed the world to consist was able to fill space entirely, or whether it consisted of particles. So far as our senses tell us, water or glass is a continuous whole; whereas sandstone or snow consists of particles which can be seen by the unaided eye. The question was whether water, if it were possible to magnify it enormously, would not be seen to consist of minute particles, similar to those seen in sandstone, except that the water - particles would be mobile. On the whole, the verdict was that matter must consist of particles.

Down to the Middle Ages the question still occupied men's minds. In the time of King Charles II., who, by the way, was the founder of the Royal

Knowledge, his apothecary, N. Lefèbure, stated clearly the arguments adduced by those who preferred the idea of a continuous instead of a discrete structure of matter; he wrote: "If you ask of what a body is composed, you will be told that that has not yet been settled by the schoolmen; that if it is a body it must have quantity, and therefore must be divisible; it is clear that bodies must consist either of things divisible or indivisible—that is, either of points or of parts. Now a body cannot consist of points, because a point is indivisible, for it has no quantity; and a point cannot convey quantity to a body, seeing it has no quantity in itself. It must, therefore, be concluded that bodies must be composed of divisible parts; but the objection to this view is that if this is so we ought to know whether the smallest part of such a body is divisible or not. If it is divisible, then it cannot be the smallest part, because it can be divided into others still smaller; and yet, if this smallest part is indivisible, there would always be the same difficulty, seeing it would be without quantity, and could not convey quantity to a body, not having it itself; for divisibility is the essential feature of quantity." The argument may be summed up in the Latin saying, De minimis non est disputandum.

In 1804 John Dalton revived the atomic hypothesis to explain the fact that when elements unite they do so in definite proportions; and when they form more than one compound with each other the elements are present in multiple proportions. To give an instance: carbon burns in oxygen; if there is plenty of oxygen, twelve parts of carbon are added to thirty-two parts of oxygen; if there is excess of carbon, then the compound contains only sixteen parts of oxygen for each twelve parts of carbon. Dalton's explana

tion was that in the first compound, carbonic-acid gas, one atom of carbon is united with two atoms of oxygen; whereas in the second, one atom of carbon has united with one atom of oxygen. This hypothesis proved of the greatest use; it gave definiteness to chemical conceptions; and, indeed, without it chemistry could not have developed. Still, it was a theory; no one had seen an atom; nor was there any direct evidence of the existence of atoms.

It came to be evident, about the middle of the last century, that, in order to explain certain facts connected with the relative weights of gases, matter must not merely consist of atoms, but that these atoms must have the power of uniting in small groups. In forming a compound, indeed, this must be so; for instance, carbonic-acid gas must consist of one atom of carbon which, along with two atoms of oxygen, forms a small group of three atoms. The novelty of the conception was in the notion that oxygen itself, in the state of gas, as it exists, for example, in the air, consists of small groups of atoms; in this case, two. To such small groups of atoms was given the name molecules. A molecule is that portion of a substance which can exist in the free state, as oxygen does in air. An atom generally exists in combination; but atoms may, and sometimes do, exist separately; in which case they also are termed molecules. Now, can molecules be seen? Is their existence a mere assumption? The answer to that question is: no, they cannot be seen; but artificial molecules can be made which correspond so closely in their behavior to real molecules that the existence of real molecules is practically certain. Moreover, although no one has ever seen a molecule, still the track of a molecule moving through space has been seen; and just as Robinson Crusoe was right in inferring the existence of Man Friday from his footstep imprinted in the sand, so the real existence of a molecule may just as certainly be inferred from the track it leaves. How that has been done we shall now proceed to explain.

Our atmosphere consists of a mixture of nitrogen and oxygen, together with small quantities of other gases, of which argon is present in largest amount. Air

is somewhat lighter than oxygen; while oxygen is sixteen times heavier than hydrogen, air is nearly fourteen and onehalf times as heavy. Now, the atmosphere presses on the surface of the earth, owing to its weight; the pressure at the level of the sea is about fourteen pounds on each square inch; it is generally measured by the height of the barometer, which, at sea-level, is thirty inches in fine weather. If we ascend a hill, the barometer falls; there is no longer so much air pressing on the earth, for there is less air above us. To halve the pressure we should require to ascend nearly four miles; this is not quite so high as the highest of the Himalayan Mountains. There would then be as much air below us as above us. Suppose now that the atmosphere consisted not of air, but of hydrogen; how high would it be necessary to ascend in order to halve the pressure? Evidently, in order to have the same weight of hydrogen pressing on us as we have air, the atmosphere would have to be fourteen times as high; and to halve the pressure we should need to ascend, not four, but fourteen times four, or fifty-six miles.

The pressure of a column of gas, be it air or hydrogen, depends evidently on two things: first, on the relative weight of the molecules of the gas; and, second, on the number of molecules in the layer of gas that is pressing on us. It is supposed, in the calculation which we have just made, that when the number of molecules is equal, then the pressure is equal; and this can be easily proved. So that a quart of air, at four miles up, would contain as many molecules as a quart of hydrogen at fifty-six miles up. From this it follows that if we knew the height at which the density of gas would be halved, and if we also knew the number of molecules of air in a quart at the height where its density is halved, we should be able to calculate the number of molecules in a quart of hydrogen.

Measurements of this kind have practically been made by Jean Perrin, one of the professors in the University of Paris; and they constitute one of the most remarkable feats that have ever been accomplished. Perrin made what may be called "artificial molecules," by pouring a solution of gamboge, the ordinary

water-color paint, into water. The gamboge separates as a milky cloud; and, seen under a microscope, the cloud consists of small, round particles of different sizes. Months were spent in separating out a lot of uniform size. This was done by a method of settling them; the larger grains settle more quickly than the smaller ones. But the process was slow, before a sufficient number of equal size had been collected. Next, the density of the grains had to be found; this was not difficult; a known volume of the suspended grains was weighed in a flask, and the weight of the solid in the water was found by evaporating off the water and weighing what remained. To find the diameter of a grain was not quite so easy; it was done by allowing some of the suspension to dry up; a lot of grains, which could be counted, often lay in a row the length of which could be measured. Of these grains, 125,000 would lie along an inch; and each grain was about one and a fifth times as heavy as water.

The next step was to make a kind of atmosphere of these grains, and to find out at what height their number would be halved. The emulsion was accordingly put into a small glass dish and placed below a photographing microscope. The microscope was focused on one of the lower layers, and a photograph was taken; naturally, only those in focus appeared on the plate. The microscope was then shifted to a known distance, so as to take a picture of the grains higher up; and this process was repeated until the numbers at different heights in this "atmosphere" of gamboge granules had been measured. Now we come to a sum in proportion. The height at which the pressure of a gas is halved is in inverse proportion to the density; thus, as already stated, because hydrogen is fourteen times less dense than air, it is necessary to go fourteen times as high in an atmosphere of hydrogen before its density is halved. So that by comparing the height required in the "atmosphere " of gamboge with that required for hydrogen or for air, their relative densities are determined; and as it is known that equal numbers of molecules, at the same temperature, exert the same pressure, and as the number of particles of gamboge in a known volume had been counted, it follows that

the number of particles-that is, of molecules-in the same volume of air or of hydrogen can be reckoned, for it is identical. An ordinary thimble holds about three cubic centimeters; the number of molecules of air which fills it is expressed by one followed by twenty naughts, or a hundred million million million. And knowing the relative weights of the same volume of hydrogen and of gamboge particles, exerting the same pressure, and at the same temperature, the weight of a molecule of hydrogen can be calculated; there are four followed by twenty-six naughts in a grain weight.

Such figures convey little; they only show us what extraordinarily small things there are in this world of ours. But the fact that "visible molecules," as we may term these particles of gamboge, behave in a manner exactly similar to molecules of gases such as hydrogen, or of the oxygen and nitrogen of which air consists, makes it absolutely certain that such invisible molecules really exist, and that their real, not merely their relative, weights can be determined.

So much for molecules; now for atoms. Every one has heard of radium, the wonderful metal discovered by Madame Curie. Among its other remarkable properties it has the one of breaking up, or disintegrating; one of its products is a gas named niton, and at the same time it expels an atom of helium, a gas discovered by the writer of this article in 1895. The breaking up of an atom of radium is accompanied by a kind of explosion, the result of which is that the atom of helium is shot off with enormous velocity indeed, at a rate of about 12,500 miles a second, which is about the fifteenth part of the velocity of light. The breaking down of radium into helium was the first known instance of the decomposition of a chemical element, or of one element changing into another; it was discovered by Mr. Frederick Soddy and the writer in 1903. The rate of motion of the helium atom thus expelled was first measured by Professor Rutherford. To Mr. C. T. R. Wilson, of Cambridge, we owe the astonishing feat of mapping the trail of the moving helium atom; and we shall try to give a description of how this wonder was accomplished.