IMEASUREMENTS OF INFINITESBLVL QUANTITIES OF 



SUBSTANCES.^ 



By Sir William Ramsay. 



Our investigation of rare gases necessarily brought ns 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 tht^ 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 cjuantity 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 in 2,700. 



Later, when we succeeded in obtaining the congeners of argon — neon, 

 ki-ypton, 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 ({uantity at our disposal did not allow 

 us to weigh more than 7 cubic centhneters ; 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 kr\^:)ton 

 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 cjuantities are very small, those of the radio- 

 active products are much smaller. In the first place, radium is not 

 found in large quantities; and owing to the slowness of its disintegra- 

 tion, which continues for thousands of years, we have only minmial 

 quantities of these substances at our disposal. Let me remind you, 

 gentlemen, that half of the life of radium goes back 1 ,700 yea.rs, that 

 it is disintegrated into emanation and helium, and that even with 



• Lecture before the Societ6 franQuise do I'hysique, Session of April 20, 1911. Translated by permission 

 from Journal de Physique, series 5, vol. 1 (June, 1911), pp. 429-442. 



219 



