439 
of Edinburgh, Session 1879 - 80 . 
For it is to be remarked that all reckoning of temperature is elimi- 
nated from the second member of the formula, and that, in our use 
of it, Eegnault’s normal thermometer has merely been referred to for 
the values of pK and of 1 - <r, which correspond to stated values of p. 
The arbitrary constant of integration, t Q , is truly arbitrary. It will 
be convenient to give it such a value that the difference of values of 
t between the freezing-point of water and the temperature for which 
p is equal to one atmo shall be 100, as this makes it agree with the 
centigrade scale in respect to the difference between the numbers 
measuring the temperatures which on the centigrade scale are 
marked 0° and 100°. Indirectly, by means of experiments on 
hydrogen gas, this assignation of the arbitrary constant of integration 
would give 273 for the absolute temperature 0° C., and 373 for that 
of 100° C, as is proved in p. 56 of the article on “Heat,” in the 
Encyclopaedia Britannica. Meantime, as said above, we have not 
the complete data from direct experiments even on water-steam for 
graduating the water-steam thermometer ; but, on the other hand 
we have, from experiments on air and on hydrogen and other gases, 
data which allow us to graduate indirectly any continuous intrinsic 
thermoscope according to the absolute scale. By thus indirectly 
graduating the water-steam thermometer, we learn the density of 
steam at different temperatures with more probable accuracy than 
it has hitherto been made known by any direct experiments on 
water-steam itself. 
Merely viewed as a continuous intrinsic thermoscope, the steam- 
thermometer, in one or other of the forms described above to suit 
different parts of the entire range from the lowest temperatures to 
temperatures somewhat above 520°, is no doubt superior in the 
conditions requisite for accuracy to every other thermoscope of any 
of the different kinds hitherto in use ; and it may be trusted more 
surely for accuracy than any other as a thermometric standard 
when once it has been graduated according to the absolute scale, 
whether by practical experiments on steam, or indirectly by experi- 
ments on air or other gases. In fact, the use of steam-pressure 
measured in definite units of pressure, as a thermoscopic effect, in 
the steam thermometer is simply a continuous extension to every 
temperature, of the principle already practically adopted for fixing 
the temperature which is called 100° on the centigrade scale; and 
