228 Scientific Intelligence. 



value of the refractivity of the mixture which was about 3 per 

 cent higher than the observed value, the difference being of an 

 inverse character to that of a mixture of oxygen and nitrogen, 

 as was to have been anticipated. 



A third experiment made with " artificial air " consisting of a 

 mixture of 19*13 percent carbon dioxide and 19*29 percent of 

 oxygen, gave a result which was less than that calculated. The 

 difference was not great, but was believed to considerably exceed 

 the error of the several determinations. 



In regard to the theoretical point involved, the authors make 

 the following remarks : 



"The coefficient of compressibility of hydrogen is too small, 

 while that of other gases, such as oxygen and nitrogen, is too 

 great. The effect of mixing equal volumes of hydrogen and 

 helium, each of which has too large a coefficient of elasticity, is 

 to cause each to occupy twice the volume that they previously 

 occupied, and to halve approximately the pressure for each. The 

 pressure is therefore lower than it would be for an absolutely 

 ideal gas, for each gas, hydrogen and helium. The sum of these 

 pressures will accordingly be too low, or transposing, the sum of 

 the volumes will be too great. The opposite argument holds for 

 air. 



Now, in considering volumes we deal not merely with the co- 

 volume, i. e., the space occupied by the molecules, but also with 

 the interstitial space inhabited by the molecules. But the refrac- 

 tive power, if Olausius's deduction from the formula of Lorentz 

 and Lorentz is correct, is a function of the dielectric constant, 

 and hence of the co-volumes of the gases. And here the dis- 

 crepancy is more easily detected than by any determination of 

 density. It must therefore be concluded that gases are not, as 

 postulated by Dalton, indifferent to one another's presence, but 

 that they modify one another's properties in the same manner as 

 do liquids, though to a different extent. This mutual action at 

 high pressures and small volumes modifies even the volume rela- 

 tions, as recently shown by Dr. Kuenen. And it must persist at 

 low pressures and large volumes, though it may not always be 

 possible to make measurements of pressure and volume accurate 

 enough to lead to its detection. The refractivity, however, seems 

 to be a means delicate enough to be used for this purpose." — 

 Proc. Boy. Soc., lxii, 225. 



18. Experiments with some Electro- Chemical Actinometers. — 

 M. H. Rigollot has recently published in the Annales de l'Uni- 

 versite de Lyon the results of an interesting series of experiments, 

 having as their object the investigation of the electromotive 

 force developed by the action of light upon one of two metallic 

 plates immersed in an electrolyte, the other being kept in dark- 

 ness. The experiments were carried on with plates of copper, 

 covered respectively with the oxide, fluoride, chloride, bromide, 

 iodide ; also with plates of tin having a surface of oxide, and of 

 sulphide, and finally of silver sulphide. The investigation 



