1903.] on Low Temperature Investigations. 419 



liquid, a polished silver ball, which had been weighed once for all in 

 liquid oxygen, was weighed in the sample of liquid air, and from the 

 relative weights thus found the density of the liquid air could be 

 approximately determined, that of liquid oxygen being 1 • 137. To 

 prevent any disturbing ebullition in the liquid-air flask in which the 

 weighings took place, and to reduce the rate of its evaporation to a 

 minimum during the course of an experiment, the substance to be 

 used was previously cooled in a supplementary vessel containing 

 liquid air and then transferred to the large flask. Substances like 

 solid carbonic acid and ice were weighed in the cool, gaseous air of 

 the vacuum vessel, and their weights subsequently corrected for 

 buoyancy. The temperatures of the densest and lightest samples of 

 liquid air were ascertained by the hydrogen thermometer, and that of 

 the others deduced by graphic interpolation. As the entire range 

 of temperature through which the bodies were cooled amounted to 

 about 200°, a degree or two up or down has no real influence on the 

 results ; the extreme range of temperature in the air samples was 

 from 83-8° to 86-1° absolute. 



Salts were employed in the form of compressed blocks. The 

 salt, previously reduced to a fine powder, was moistened with vt^ater 

 and compressed in a cylindrical steel mould under great hydraulic 

 pressure. During compression the saturated salt solution drained 

 away, and finally a cylindrical block of some 50 grammes of the 

 salt was obtained free from porosity and hard enough to allow its 

 surface to be polished. In this form salts and other materials 

 similarly treated are especially adapted for accurate specific gravity 

 determinations. After such treatment it was found that all the 

 mechanically attached water was got rid of in the case of hydrated 

 salts, and also in such as did not combine with water. In order to 

 get cylindrical blocks of the salts showing no porosity, the presence 

 of water, or rather the saturated salt solution, was found to be essen- 

 tial during the application of pressure. In the same way it was 

 found to be an advantage in compressing such a substance as solid 

 carbonic acid, to moisten it with a fluid like ether before applying 

 the hydraulic pressure. 



Recalling the work of Playfair and Joule,* which originated in a 

 suggestion of Dalton's that the volume of a hydrated salt in solution 

 was simply the volume of the water of crystallisation as ice, some 

 hydrated salts were selected, as well as some other bodies whose 

 coefficients of expansion they had determined. Substances of special 

 interest included in the list, were ice, mercury, sulphur, iodine, and 

 solid carbonic acid, the latter being particularly important as an 

 example of a solidified gas. 



The specific gravity of the actual portion of the substance weighed 

 in the liquid air was, with one or two exceptions, determined also at 



* " Kesearches on Atomic Volume and Specific Gravity," Chem. Soc. Jour., 

 vol. i., 121. 



