MEASUREMENT OF TEMPERATURE. 
181 
Changes of Zero Pressure .* * * § 
In spite of all precautions, slight changes of zero pressure were observed with all 
the air thermometers, amounting in some cases to upwards of a millimetre of mercury 
in a week ; these were probably due to surface condensation. In support of this it 
may be remarked that the vortex atom theory of gases requires the existence of an 
ah' film of swollen vortices sticking to the sides of the containing vessel; the 
phenomenon is also well attested by many experiments ; in particular, we may 
mention that of Professor Bunsen “ on the condensation of C0 3 on the surface of 
fine-spun glass.” t It is still necessary, however, to determine in what way the air 
film varies with the temperature. I am investigating this, but the experiments are 
not yet completed. For the present we may assume that it exists and see what kind 
of error it will produce. 
Let m be the mass of air enclosed in grammes; p, v, 0, its pressure, volume, and 
absolute temperature ; k the difference of the specific heats of unit mass of air; <j 
the surface density of the air film in grammes per square centim. ; and S the surface 
of the air thermometer in square centims. a is a function of the pressure and 
temperature, and also apparently of the nature of the gas, and of the surface, of its 
past history, and of the time. 
We shall have the equation 
ifeScr+f = mk = &Scr 0 +^f-°.(2) 
First approximation, 6 = 
we have 
Vf o 
vf 
Solving this to a second approximation, <x being small, 
neglecting other corrections. 
Let p v p 0 , be the pressures corresponding to 100° and 0° C. respectively, then, 
according to R,egnault, l PilPo = 1'3665 ; according to Balfour, Stewart, § 
Pi / p 0 = 1 ‘ 3673. In R,egnault’s experiments S/m = 400 roughly. In Balfour 
Stewart’s case S/m = 1060, so that, if we assume cr l — <x 0 = '000001 gramme per 
square centim., we shall derive, both from Regnault and Balfour Stewart, the 
value Pi/Pq — 1'3659, corrected for surface condensation; also, in the case where 
S/m = 2400, as in my air thermometer (No. III.), we should expect to find the value 
* Similar changes were observed by Regnault. 
t ‘ Phil. Mag.,’ Series 5, vol. 17, p. 161. 
X Vide ‘ Mem. Inst.,’ tom. 21. 
§ Vide ‘ Phil. Trans.,’ 1863. 
