818 Mr. F B. Young on the 



proportion of molecules having less than a given velocity k, 

 which is itself less than the mean velocity o£ translation K, 

 diminishes as k is taken further from K, but the rate of 

 diminution is less rapid as (K— -&) becomes greater. This 

 matter will, however, probably be further investigated. 



G. Effects of Impurity. 



Dwelshauvers-Dery (17) maintained a tube of carbon 

 dioxide at various temperatures above C and then allowed 

 the tube to cool. A zone of emulsion appeared in the neigh- 

 bourhood of the point at which the meniscus had disappeared, 

 and as the temperature to which the tube was raised became 

 higher, the zone of emulsion which was formed on cooling 

 became broader. The emulsion, however, was not uniform 

 throughout the tube even when the temperature had been 

 raised 20° above C . He inferred from his observations of 

 carbon dioxide that the two states of the substance persisted 

 even to the highest temperature reached. The zones of 

 opalescence were regarded by him as zones in which mutual 

 diffusion of the gasons and liquidons had occurred, so that 

 when the temperature was lowered a cloud was formed by 

 the separation of the two kinds of molecules. 



Andrews (18), however, has obtained similar opalescent 

 zones in working with mixtures of carbon dioxide and small 

 proportions of nitrogen, though under rather different experi- 

 mental conditions. The similarity of these results suggested 

 that the phenomena of opalescence might be closely associated 

 with, if not entirely dependent upon, the presence of impurity. 

 It was partly with the object of testing this that special 

 efforts w r ere made to free the ether from impurity. 



It was found, however, that the formation of opalescence 

 is retarded rather than assisted by the presence of impurity. 

 As the ether was obtained purer it more readily showed signs 

 of general opalescence when its temperature was raised. 

 When tubes VII. and XIV. were together raised in tempera- 

 ture slightly above 6 C , some time after the opalescence in 

 XIV. had become general, tube VII. showed only a slight 

 opalescence throughout the tube, whilst a denser cloud 

 appeared in the neighbourhood of the point at which the 

 meniscus had faded. If the ether was stirred, however, both 

 tubes immediately appeared equally opalescent. The influence 

 of traces of impurity upon the opalescent phenomena is pro- 

 bably indirect. If a trace of gas is present in the upper 

 part only of the tube, it produces, by its partial pressure, a 

 difference of density of the ether which may be so great that 



