556 Intelligence and Miscellaneous Articles. 



heat, fused or simply dried at 100° C. before weighing them. The 

 densities were taken by means of a specific-gravity bottle, at tem- 

 peratures between 0° and 100° C, in an apparatus resembling that 

 for determining the boiling-point of thermometers. The determina- 

 tion of the densities was made twice for each solution, at intervals 

 of about six months. The measurement of the refractive indices 

 was effected by receiving a pencil of parallel rays on a prism con- 

 taining the liquid investigated, and determining in each case the 

 two minima of deviation. The prisms employed were constructed 

 of angular flasks, each provided with a ^lateral aperture, on which 

 plane glass was hermetically fixed. 



Two series of experiments were made, thus : — one at the ordinary 

 temperature, the principal object, of which was to see if Biot and 

 Arago's law was applicable to solutions ; the second at temperatures 

 increasing from 10° to 95° C, the object of which was to determine 

 the variations of the index and of the refracting-power with the 

 temperatures. 



In the first series, the necessity of working at constant tempera- 

 tures led me to fit up the apparatus in a cellar at the observatory, 

 where the daily variations are very small, and to avoid as much as 

 possible the other ordinary sources of heat. Hence it was that I 

 used as principal source of light, in determining the indices, the 

 light of a Geissler's hydrogen-tube ; and I used similar tubes, fur- 

 nishing a bright light and a very feeble heat, to read the ther- 

 mometer and the graduated circle. 



In the second series of experiments, the prism containing the 

 solutions was placed in an oven closed laterally with parallel glass 

 plates, forming a jacket in which circulated the vapour of water, 

 of alcohol or ether, or vapours of mixtures of these liquids in various 

 proportions. The condensed liquids returned to the boiler in such 

 a manner as to keep up a continual circulation of liquid and vapour, 

 and thus preserve a constant temperature. 



One hundred and twenty-three solutions of different salts in 

 water, and moreover some simple liquids, such as water, ether, 

 alcohol, benzole, sulphide of carbon, were thus investigated. The 

 numerical results obtained lead to the following conclusions : — 



(1) The refractive index varies considerably with the temperature. 

 In the interval from 10° to 95° C, the variation of the index for 

 saline solutions always amounts to hundredths. 



(2) The variation is greater the more concentrated the liquid. 



(3) The refractive power of saline solutions diminishes as the 

 temperature rises. This diminution is about 0*001 for all the solu- 

 tions I have examined between 10° and 95° C. The mean coeffi- 

 cient which represents this variation of the refracting-power most 

 frequently diminishes as the concentration of the solution in- 

 creases ; sometimes it remains stationary; at others, on the contrary, 

 it also increases ; but in all cases it varies much less than the index 

 with the concentration of the liquid. 



(4) The dispersion diminishes when the temperature increases : 

 the difference between the indices of the rays of a and b of the 



