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Proceedings of the Boyal Society 
means, to the highest for which glass remains solid. This, I believe, 
may be done by avoiding the objectionable expedient adopted by 
Pouillet and Regnault, of allowing a portion (when high tempera- 
tures are to be measured the greater portion) of the whole gas to be 
pressed into a cool volumetric chamber, out of the thermometric 
chamber proper, by the expansion of the portion which remains 
in ; and instead fulfilling the condition, stated, but pronounced 
practically impossible, by Kegnault (“ Experiences,” vol. i. pp. 168, 
169), that the thermometric gas “ shall like the mercury of a mercury 
thermometer be allowed to expand freely at constant pressure in a 
calibrated reservoir maintained throughout at one temperature.” I 
have accordingly designed a constant pressure gas thermometer to 
fulfil this condition. It is represented in the accompanying drawing, 
and described in the following extract from the article referred to : — 
The vessel containing the thermometric fluid, which in this case 
is to be either hydrogen or nitrogen,* consists in the main of a glass 
bulb and tube placed vertically with bulb up and mouth down ; 
but there is to be a secondary tube of much finer bore opening into 
the bulb or into the main tube near its top, as may be found most 
convenient in any particular case. The main tube which, to dis- 
tinguish it from the secondary tube, will be called the volumetric 
tube, is to be of large bore, not less than 2 or 3 centimetres, and is 
to be ground internally to a truly cylindric form. To allow this to 
be done it must be made of thick, well-annealed glass like that of 
the French glass-barrelled air-pumps. The secondary tube, which 
* Common air is inadmissible, because even at ordinary temperatures its 
oxygen attacks mercury. The film of oxide thus formed would be very incon- 
venient at the surface of the mercury caulking, round the base of the piston, 
and on the inner surface of the glass tube to which it would adhere. Besides, 
sooner or later the whole quantity of oxygen in the air must be diminished to 
a sensible degree by the loss of the part of it which combines with the mercury. 
So far as we know, Regnault did not complain of this evil in his use of 
common air in his normal air thermometer nor in his experiments on the 
expansion of air (“ Experiences,” vol. i.), though probably it has vitiated his 
results to some sensible degree. But he found it to produce such great irregu- 
larities when, instead of common air, he experimented on pure oxygen, that 
from the results he could draw no conclusion as to the expansion of this gas 
(“ Experiences,” vol. i. p. 77). Another reason for the avoidance of air or other 
gas containing free oxygen is to save the oil or other liquid which is inter- 
posed between it and the mercury of the manometer from being thickened or 
otherwise altered by oxidation. 
