MEASUREMENTS OF INFINITESIMAL QUANTITIES OF SUBSTANCES.1 By Sir WILLIAM RAMSAY. Our investigation of rare gases necessarily brought us to measure their densities, in order to be able to draw a conclusion touching their atomic weights. Until then (1895) vessels with a capacity of several liters were employed, e. g., Regnault in his classic experiments made use of balloons containing about 2 liters, while Lord Rayleigh employed vessels of the same capacity. Unable at the beginning of our researches to separate from the atmosphere more than 200 cubic centimeters of argon, we were compelled to determine its density with a much smaller quantity, about 160 cubic centimeters. It is obvious, however, that even this quantity is capable of giving a satisfactory result; for the weight of argon ascertained by the balance was 0.27 gram, and even with a balance the sensitiveness of which does not exceed 0.1 milligram, the error is no more than 1 part în 2,700. Later, when we succeeded in obtaining the congeners of argon-neon, krypton, and xenon-which form a minimal fraction of the atmosphere, we became bolder. We weighed only 32 cubic centimeters of neon at a pressure of half an atmosphere; its weight was about 0.011 gram. As to krypton and xenon, the quantity at our disposal did not allow us to weigh more than 7 cubic centimeters; but their greater densities permitted the attainment of an equal degree of accuracy, for the weight was 0.015 gram. The error did not exceed 1 or 2 per thousand. We also attempted to determine the specific volumes of krypton and xenon in the liquid state. We constructed capillary tubes in which the gases were liquefied at a low temperature, and we succeeded in measuring quantities such as 0.006 cubic centimeter. But although these quantities are very small, those of the radioactive products are much smaller. In the first place, radium is not found in large quantities; and owing to the slowness of its disintegration, which continues for thousands of years, we have only minimal quantities of these substances at our disposal. Let me remind you, gentlemen, that half of the life of radium goes back 1,700 years, that it is disintegrated into emanation and helium, and that even with 1 Lecture before the Société française de Physique, Session of April 20, 1911. Translated by permission from Journal de Physique, series 5, vol. 1 (June, 1911), pp. 429-442. 0.50 gram of bromide of radium, the total quantity of emanation can never exceed 0.1 cubic millimeter. Let us first see what are the means at our disposal of performing these operations. We may calculate: With a good chemical balance.... With an assay-balance... With Nernst's micro-balance.. With the micro-balance constructed by Whytlaw- 10+=0.0,001 10-5=0.0,000,01 3 X 100.0,000,000,003 The spectroscope permits the discovery of helium. 2 X 10-10-0.0,000,000,000,2 The electroscope................ 10-11=0.0,000,000,000,01 10-12-0.0,000,000,000,001 Everyone is aware how the electroscope has become a much-used means of determining the presence of radium and thorium in rocks. To it M. and Madame Curie owed their discovery of radium, and those who determine the content of rocks in radium confidently distinguish between samples which contain 2.3 X 10-12 and those containing 2.4 X 10-12 gram per gram of ore. Within the last few years we have been devoting our attention to the emanation into which radium changes spontaneously. Owing to the courtesy of the Vienna Academy, I have had at my disposal more than half a gram of bromide of radium, and Mr. Whytlaw-Gray and I have succeeded not only in measuring the quantity of gas which it continually gives off in the gaseous state, but even in determining its volume in the liquid state; in freezing it by cooling it with liquid air; in measuring the wave-lengths of the rays of its spectrum; and in weighing a quantity not exceeding one-tenth of a cubic millimeter. But that is not all. Messrs. Cuthbertson and Porter, my colleagues at University College, have invented an apparatus enabling us to determine the index of refraction of that minimal quantity. There is a proverb in English, "Cut your coat according to your cloth," and its parallel in French seems to be, "Regulate your mouth by your purse." Our purse contained but little of the noble element, and perhaps this lecture corresponds to that minimal quantity; but I rely upon your indulgence, "making of an egg an ox." This bromide of radium, dissolved in water, was contained in a small glass bulb sealed onto a Toepler pump. You know, gentlemen, that under its influence water is decomposed into hydrogen and oxygen. We actually obtained weekly almost exactly 25 cubic centimeters of detonating gas. There is always a small excess of hydrogen, doubtless by reason of the formation of peroxide of hydrogen. This excess is very useful, for it gives after explosion a bubble of hydrogen which carries the emanation and permits of transferring it into vessels for experiment. Moreover, hydrogen is not condensed at the temperature of liquid air, while the emanation is deposited on the walls of the vessel in a solid state. Consequently, we can easily separate the two, removing the hydrogen with the pump. The emanation then remains entirely pure. I shall begin by giving you an idea as to how we set about measuring the volume of the emanation. In the first place, one must avoid all contact between the emanation and the grease of the pump-tap, for fear that the resulting carbonic acid may make the gas impure. We avoid all danger of this kind by sealing the explosion-tube with mercury; but in order to guard against this contamination, we leave the hydrogen with the emanation for three hours in a small tube whose upper part contains some caustic potash melted upon the walls. This period having elapsed, the products of the disintegration of the emanation have reached their maximum, for radiums A, B, and C have but a short life, changing into D, which is without radio-activity. It being necessary, for reasons which will appear later on, to measure the y rays at this juncture, with the aid of an electroscope we determine the emanation's power of discharge. In figure 1 we see the small tube a. We see also a reversed siphon closed by a kind of glass cap. The cap being raised and the tube placed above, the gases enter the apparatus through the capillary tube b, narrow at the upper end, to prevent the mercury from entering too rapidly. h FIG. 1. But before introducing the gas it is necessary to empty the apparatus. Opening the tap and lowering the reservoir f we connect the pump with the apparatus. We remove all air and by means of the reversed siphon admit some hydrogen, which must remain for a night in the apparatus in order to displace atmospheric air adhering to the walls. This displacement is accelerated by heating in the flame the tubes k, i, j, l, m. In the morning the apparatus is again pumped empty. The reservoir f is raised in order to make the mercury ascend above the taph, and the tap is shut. At noon the measurements of radioactivity will have been made, and we introduce the hydrogen with the emanation, replacing the cap, which, sealed with mercury, has no need of grease to make the joint air-tight. The gases can not come into contact with the tap, which is protected by the mercury. You can see a tube i containing some pieces of caustic baryta. This arrangement is for two purposes: First, to dry the gas; secondly, to remove every trace of carbonic acid which may have escaped the action of the potash. The gas is left in the tube for half an hour to make sure that neither moisture nor carbonic acid remain. In there is a cone of blotting-paper, which is moistened with water. Pouring in liquid air, it forms a vessel tight for liquid air, because it consists of ice. The tube which it surrounds is so cooled that the emanation freezes in its interior and is deposited upon the walls. We may now open the tap h, for the emanation can no longer be contaminated, and the hydrogen is pumped off. But even at the temperature of liquid air the emanation possesses some little vapor pressure, and, consequently, a certain quantity accompanies the hydrogen. To take this quantity into account, its radio-activity is next determined, and by comparing it with that of the gas before its introduction into the apparatus the loss is ascertained, and consequently the quantity remaining in the apparatus. Having removed the hydrogen as far as possible, we raise the reservoir f in order to protect the tap h against attack by the emanation, and heat the tube j. The emanation is volatilized. Raising the reservoir d, it is compressed into the capillary tube m, where its volume is measured. But this tube must be chosen of such a diameter that its volume can be determined at a pressure little removed from that of the atmosphere. If we measure under too low a pressure, the correction for the capillarity, which may rise to two centimeters of mercury, becomes too great. However, it is difficult to estimate the magnitude of the correction, for we have been unable to find constant results for the capillarity; it seems to be influenced by the state of electrification of the mercury, due to the presence of the emanation. But that is of little consequence when the measurement is made at a pressure near 760 millimeters, for we may almost disregard a pressure of 5 millimeters, which does not exceed 0.7 per cent of the total pressure. To give you an idea of the exactness of this measurement, allow me to state the result of a recent experiment by which we ascertained that a sample of helium had a volume of 0.042 cubic millimeter; the length of the tube measured was 20 millimeters, which permits the attainment of an approximation of 1/200. A platinum wire sealed through the top of the capillary tube makes it possible to test the purity of this gas by passing through it an electric discharge. The other electrode consists of the mercury of the reservoir, and we may prevent the lines of mercury from being seen by solidifying it with a paper cone filled with liquid air, surrounding the tube. We have compressed the emanation so as to liquefy it; the tube containing it is of thick glass and can resist a considerable pressure. We have cut it off and mounted it in a compression apparatus like that of Amagat. The emanation liquefies at the ordinary temperature at a pressure of about 10 meters of mercury. The volume of gas, which was 0.1 cubic millimeter, was reduced to 0.00025 cubic millimeter; it occupied about 0.24 millimeter of the length of the capillary tube. Naturally, we used a microscope to observe it. However, it was easy to observe its properties. The emanation is colorless like water when seen by transmitted light. Seen by reflected light, it makes the tube phosphorescent, and its color depends upon the nature of the glass. In silica it emits a white light, in sodaglass a lilac color, in potash-glass the color is greenish blue. When the emanation is liquefied in soda-glass its appearance recalls that of a cyanogen flame, being at the same time bluish and pink. This gas when cooled to -71° becomes opaque and solidifies. A striking change of color is noticeable; the solidified emanation causes the glass to emit a brilliant steel-blue glow like a small electric arc. At a still lower temperature the color changes to yellow, and in liquid air it becomes orange red. When the temperature rises, the changes of color come in inverse order. Though its brilliancy is very intense, I scarcely think that its use can rival modern methods of illumination. We have succeeded in measuring the volume of this rare liquid, and knowing, as we shall see later, that the density of the gas is 112.5, we may calculate the density of the liquid. It is very heavy, 5.7 times heavier than water. I have hitherto designated this gas by the name given it by Messrs. Rutherford and Soddy. But it doubtless belongs to the series of inactive gases, and there are already three emanations-that of radium, of thorium, and of actinium. The expression "radium emanation" is not a very happy one, and we had to seek a name to indicate one of the striking properties of the gas and at the same time to recall its congeners of the argon series. We coined the name of niton, signifying "shining." It is true we paid no heed to the scruples of the purists, who forbid the addition of a Greek ending to word of Latin origin. My excuse is that it was a generally accepted custom among the Greeks to adopt Latin words (e. g., σovdàprov, dqvả, ριον, πραιτώριον, Κήνσος, and numerous others). a Dr. Collie and I succeeded in 1904 in measuring the wave lengths of some of the spectral lines of niton. In collaboration with Mr. Cameron other attempts were made. Mr. Watson made in my laboratory a thorough study of this question with niton purified by me. According to Prof. Hicks, this spectrum presents close analogies with the spectra of the inactive gases. Being itself an inactive gas, there were many probabilities for its belonging in that series of ele ments. Several attempts have been made to determine precisely the atomic weight of niton. I shall confine myself to mentioning the experiments by means of diffusion of Curie and Danne, of Bumstead and Wheeler, of Rutherford and Miss Brooks, of Makower, of Chaumont, |