of Superheated Vapours. 405 



H is the difference (reduced to 0°) of the levels of the mer- 

 cury in the tube at each experiment, and so the proper object cf 

 the measurements. In the calculations, H is given in milli- 

 metres. 



v 3 R, and V are obtained in the following manner from the 

 mercury-weighings : — 



If we imagine the tube divided into two exactly equal parts, 

 and if W 2 is the volume of each part at 0° expressed in the 

 weight of mercury at 0°, at the temperature t of observation it 

 becomes W 2 (l+ j3t), if /3 is the coefficient of expansion of the 

 glass. If, now, during the observation made for the temperature 

 t the weight of the mercury in the vapour-half of the tube is 

 found to be W,, this occupies the space W^l+yt), where y is 

 the known coefficient of expansion of mercury at that tempera- 

 ture. The vapour-volume v at the temperature / is evidently 

 equal to the difference, 



c=W a (l +00-^(1 + 7*). 



But, from the mercury- weighings mentioned in the preceding 

 section, we obtain 



_ a? l + 7 fl 

 2 ~2 1+/36 



W 1 (l+ 7 0=W Q (l+/30-2/[l + 7^.][l+^(/-6' 1 )],ifforthe 

 temperature t we first select exactly that temperature of obser- 

 vation at which we at the same time noted down the position of 

 the mercury about the diamond-strokes. 



Just so 



V=W 2 (l+/30-(W-W 1 )(l+70; 

 and, finally, 



where « is the well-known value 0-003663. 



It will at the same time be seen that all the space-determina- 

 tions are effected with the same unit, mercury at 0°. 



For any other /, v and V acquire other values only so far as 

 W, changes with t ; while W 2 and W remain constant for all 

 values. On account, however, of the cylindricity of the part 

 of the tube which here comes into consideration, we must put 



rfW, L dR 



_ r= « const _ ; 



that is, according to No. 5 of the above mercury-weighings, re- 

 membering that H is given in height of mercury at 0°, 



dW. ,_ , „,dH. , dR 



