108 
DR. M. W. TRAVERS, MR. G. SENTER, AND DR. A. JAQUEROD 
The constant-volume hydrogen .scale a])peared to approximate closely to the al)solute 
scale of temperature, and the gas could be employed in any measurements to which 
thermometers could be aj)plied. For this reason the Comite International adopted 
as the normal scale of temperature the scale of a constant-volume hydrogen ther¬ 
mometer, in which the pressure at the ice-point was 1000 millims. of mercury. 
Ptecently, however, we have succeeded in reaching temperatures at which the 
vapour pressure of hydrogen itself is extremely small, and which cannot be investi¬ 
gated by means of the hydrogen thermometer with any degree of accuracy (see p. 179). 
Furtlier, it has now been ascertained that at high temperatures hydrogen will reduce 
glass, porcelain, and even silica; and as the gas diffuses readily through the walls of 
2 )latlnum vessels, its application in this direction must be considered limited. 
It has been pointed out by one of us (“ ExjDerimental Study of Gases,” p. 156) that 
pure helium appears to be a much more perfect thermometric substance than hydrogen. 
It is chemically inactive ; and, so far as we can predict, is incapable of undergoing- 
dissociation at high temperatures. Hence it is jjrobable that measurements of high 
temperatures, made by means of quartz thermometers filled with helium, are alone 
to be relied upon. For the measurement of low tenqoeratures it j^i'esents similar 
advantages ; for as it appears probable that its critical point lies below 12°abs,, it may 
be considered as remaining a very perfect gas down to the lowest temperatures which 
it has hitherto been possible to reach. 
Further, as the following results show, the hydrogen and helium scales agree closely 
between 0° and 100° C., and consequently it may be assumed that at temperatures 
above 0° C. the difference between the thermo-dynamic scale of temperature and the 
scale of a constant-volume thermometer filled with one of these gases is small. At 
lower temperatures the divergence between the two scales is small, but the contraction 
of the helium being always less than that of the hydrogen indicates that the former 
remains the more perfect gas. Unless some hitherto unknown olqection is attached 
to the use of the helium thermometer, the helium scale should replace the hydrogen 
scale as the normal scale of temperature. 
■3. Previous Measurements of the Pressure Coefficient of Hyrlrogen and Helium. 
The first step in the sei'les of researches on which w-e have embarked consisted in 
the determination of the pressure coefficients at con.stant volume of hydrogen and 
helium between the melting-point of ice and the boiling-point of water under standard 
pressure, w-hlch, in the ba.sement of University College, is equivalent to 759-56 millims. 
of mercury at 0° 0. The coefficient for hydrogen, at a j^ressure of 100 millims. of 
mercury at the ice-point, is given l)y Chappuis (“ Travaux et IMemoires du Bureau 
International des Folds et Mesures”)as 0-00366254, the mean of seven determinations, 
of which the highest is 0-00366271 and the lowest 0-00366231, and on this deter- 
