124 KEi'OKT— 1886. 



The remarkable closeness with which these numbers of Hertz and of 

 Ramsay and Young agree is a striking proof of the applicability of the 

 thermal relation R,'=R+ c(i'- f), explained in ' Phil. Mag.' Jan. 1886, to 

 the determination, with considerable accuracy of data which almost baflBe 

 direct experimental treatment. 



Crookes, in tlie ' Chemical News,' July 16, 188G, p. 28, writing of the 

 mercury left in his radiant-matter tubes even at great exhaustions, 

 says that, although in the cold it is impossible to get an induction spark 

 through the tube, the interior of it being absolutely non-condnctmg, yet 

 on heating the tube with aBunsen flame, keeping the coil going, suddenly 

 the current passes, lighting up the inside of the tube with a greenish 

 blue light, in which the spectroscope shows strong mercury lines. The 

 tube on cooling becomes non-conducting again. 



This shows, according to Crookes, that in such very highly exhausted 

 vacuum-tubes there is plenty of mercury present, not as vapour, but con- 

 densed on the metallic poles or on the inside of the glass. But a com- 

 plete blockade may be established, as Crookes explains in this paper, 

 whereby during the exhaustion of the vacuum-tube no mercury can 

 enter ; the blockade is effected by interposing between the vacuum-tube 

 and the mercury a tube containing freshly heated sulphur and iodide of 

 sulphur packed with freshly heated asbestos, and a glass tube containing 

 copper to retain any sulphur. By this means the vacuum-tube is so 

 freed from mercury that Crookes has been unable to detect mercury 

 vapour in any of the tubes, even on heating them. 



Tables for Gonfitant Temperatures. 



The fact that for each pressure there is a corresponding temperature 

 for any volatilisable liquid, constant so long as the pi-essure is the same ; 

 and for each temperature a pressure of vapour, constant so long as the 

 tempei'ature is the same, can be utilised to secure a constant temperature 

 by boiling a liquid at constant pressure ; this is the principle of Hof- 

 mann's method of determining vapour-densities ; the constant temperature 

 being that of a given liquid boiling at the (constant) pressure of the 

 atmosphere, different liquids must be used boiling at different tempera- 

 tures to give a convenient temperature in each case. 



The substances which can be used must be few, as they must satisfy 

 the condition of being cheap, stable, and easily obtained pure. 



Now the number of constant temperatures which can thus be obtained 

 is the number of such bodies the boiling points of which can be used as 

 the constant temperatures ; the temperatures thus attainable will therefore 

 be few and far between. 



Among the liquids satisfying the conditions mentioned are carbon 

 bisulphide, ethyl alcohol, chlorobenzene, bromobenzene, aniline, methyl 

 salicylate, bromonaphthalene, and mercury. Their approximate boiling- 

 points are 46°, 78°, 132°, 155°, 184°, 222°, 280°, and 358° ; thus we have 

 eight temperatures which can be used for purposes for which a constant 

 temperature is required ; and they ai-e at fairly uniform intervals from 

 46° to 358°, applicable therefore to wide ranges of temperature. 



By the aid of the principle stated above, we can, by keeping constant 

 any pressure below 760 mm. for carbon bisulphide, obtain another con- 

 slant temperature, and in fact a whole series of constant temperatures- 



