Maintenance of Constant Pressures and Temperatures. 411 



mental test ; but in the meantime we give no further indica- 

 tion of the reasoning by which it has been arrived at, nor of 

 the way in which our equations would be modified, preferring 

 to leave the investigation in a perfectly rigid form as it now 

 stands, rather than to introduce any assumption which might 

 appear problematical, even although such an introduction 

 would both add weight to our theory, and might explain, from 

 the velocity in space of a place at midnight being greater than 

 at midday, the cause of the solar-diurnal magnetic variation. 



LXIV, The Maintenance of Constant Pressures and Tempe- 

 ratures, By Frederick D. Brown, B.Sc* 

 [Plate XIIL] 



THE great majority of the results obtained from physical 

 experiments vary with the temperature at which the 

 observations are made. The measurements of the density of a 

 substance, for example, of its refractive index, of its electric 

 conductivity, of its elasticity, of the maximum tension, and of 

 the latent heat of its vapour, all require that the temperature 

 should not vary during the observations. Hitherto many of 

 these measurements have been confined to temperatures dif- 

 fering little from that of the atmosphere ; such temperatures 

 are easily maintained constant by means of a bath of water or 

 other liquid ; but when we try to make observations at higher 

 temperatures, the means at our disposal fail us, and we find 

 that, except at certain points (such as 100°), we cannot keep 

 up the same temperature long enough to make at leisure ac- 

 curate readings of our instruments. 



Many attempts have been made to obviate this difficulty, 

 but, as it seems to me, without complete success. The ordi- 

 nary method has been to use a large quantity of water or 

 other liquid, and to keep it in continual agitation ; above 

 50°, however, the temperature of such a bath is rarely, or 

 never, rigorously constant, while the inconvenience and waste 

 of time incurred in heating the large mass of liquid to the re- 

 quired point are by no means to be neglected. 



In order to keep such a bath at a constant temperature, a 

 large number of gas-regulators have been invented. In most 

 of these a small vessel containing mercury or air is placed in 

 the bath, and arrangements are made by which the gas-supply 

 is partially shut off when the mercury expands beyond a cer- 

 tain point. Probably the most sensitive form of this kind of 

 apparatus is that recently described by M. Benoit to the French 



* Communicated by the Physical Society. 



