PHYSICS. 459 



process be slow. If it be so slow that the evaporation is allowed to 

 take place from the surfiice only, the solid mass is entirely transparent 

 Increasing the pressure to 1 atmosphere liquefies it. (C. i^., October, 



1884, xoix, TOG.) 



In hygrometry, Jamin has called attention to the unsatisfactory char- 

 f 

 acter of the ratio ^, / being the elastic force of the vapor as observed 



and F the maximum tension for that temperature, this ratio being called 

 the relative humidity, to express the quantity of vapor in the air. This 

 ratio varies with the jiroportion of vapor in the air, with the altitude 

 and the barometric pressure, and with the temperature. He therefore 



f 

 proposes the ratio - / , which measures the hvgrometric composition 



^— / 

 of the air. This value he calls the hygrometric richness. (J. Phys., 

 November, 1884, II, iii, 469.) ' 



Crova has pronounced in favor of the Saussure hair-hygrometer, and 

 says that, well made and with a good table of calibrated values, it 

 will give very satisfactory results, entirely comparable with those of 

 other instruments. He gives in his paper the details of a plan of gradu- 

 ating absorption instruments, which, is simple and apparently accurate. 

 {J. Phys., September, 1884, II, iii, 390.) 



Pernter has made a series of psych rometrical observations on the 

 Obir, 6,722 feet above the sea-level, using Wild's ventilation-hygrom- 

 eter, Eegnault's dew-point hygrometer, and Schwackhofer's volumetric 

 hygrometer. The general result obtained is that an exact formula foi 

 the psychrometer can scarcely be obtained, and that therefore we can- 

 not expect by means of the psychrometer to determine the pressure of 

 vapor to within 0.1""". {Beihldtter der Physik, viii, 31 ; Phil. Mag., May, 

 1884, V, XVII, 412.) 



LIGHT. ' 



1. Production and Velocity. 



Lecher has made an experiment to determine whether the velocity ot 

 light is affected by the motion of a current in the medium. A beam 

 of light was divided into two parts, which, after passing through two 

 parallel glass troughs, were united by an interference prism, giving the 

 usual fringes. The troughs contained a strong solution of silver nitrate. 

 and, by means of suitable silver electrodes, an electric current of six 

 amperes strength was carried in opposite diyections through the trough, 

 so that in one the electric current flowed in the same direction as the 

 light, and in the other in the opposite direction. But in no case was 

 any displacement of the fringes observed. Hence he concludes that 

 the velocity of light is not influenced by a current flowing through the 

 medium. {Nature, xxix, 559, April, 1884.) 



At the Electrical Congress held in Paris in April it was decided to 

 adopt the light emitted from a square centimeter of platinum at its 



