Correction of the Gas-Thermometer. 61 



aggregates. All the thermodynamical properties of the gas 

 may be simply expressed (as I have shown, Proc. Roy. Soc. 

 1900, vol. lxvii. p. 266) in terms of the co-aggregation- 

 volume. This method of expression is more convenient for 

 practical purposes than expression in terms of the capillary 

 pressure, since the latter is a function of both volume and 

 temperature. It is convenient to assume that c varies in- 

 versely as the nth power of 0, so that we may write 



c^oW, (29) 



where c is the value of c at . The value of the index n is 

 apparently 2 for G0 2 , but it may have different values for 

 other types of molecules. The general expression for the 

 cooling-effect deduced from (-1) on this assumption is 



SQ = (*i + l)c-&, (30) 



and the expression for the slope of the isothermals obtained 

 by plotting the product pv against p is 



d(pv)/dp=-(c-b) (31) 



For an imperfect gas like C0 2 or nitrogen, c is greater 

 than b. The gas becomes " pluperfect," like hydrogen, at 

 the point where c = b. The isothermals on thepv, p diagram 

 are straight lines, the inclinations of which to the axis of p 

 diminish as the temperature rises. This is a much better 

 approximation than might be supposed at first sight, because 

 experiment shows the isothermals to be nearly straight for a 

 considerable range of pressure and temperature. 



If we calculate the values of the constants c and b for each 

 gas from the experiments of Joule and Thomson on the 

 cooling-effect, employing the equation (28) above with the 

 value n = 2, so that SQ = 3c — b, we obtain the results given 

 in the following table : — 



Table II. — Values of Constants from Observations of 

 Joule and Thomson. 



Gas 



employed. 



Air 



xlO c . 



xio s . ! 



2-871 



10-05 



CO., 



1-887 



8-40 



, H„ 



I 



41-5 



145-3 



Q - 



Qi. 



Per atmo. (10 G c.g.s. 



0-271 

 1-368 



0147 

 0*610 



■029 - -048 



Co- 

 ca 



0-90 

 4\V> 

 201 



6. 

 c.c. 



C. 

 c p Q 373R. 



•002 O0085 



218 O0656 



10-23 -000132 



