: 
TRANSACTIONS OF SECTION A. 929 
a pressure p, to the amount at another pressure p, ts approximately constant for all 
liquids ; or 
at p, bears a constant proportion to— 
3185 
pus at Pe for all liquids and probably for all solids. 
ae | 2 
It has also been noticed as a corollary from the first law that the internal and 
total work bear an approximately constant ratio to each other for any one pressure, 
whatever the liquid. 
In considering the second part of the equation, the following relations have 
been noticed :— 
If acurve be constructed to represent the relation of temperature to pressure 
for any liquid, and if tangents be drawn to touch the curve at various points corre- 
sponding to certain temperatures, these tangents will give the rate of increase of 
pressure per unit rise of temperature, in other words, the value = for those tem- 
peratures. 
If we construct curves for a number of substances, and determine the value of 
_ for each of them at the same temperature, it isclear that the values obtained will 
differ widely, and will be greater for volatile substances than for those which are 
less volatile. But if we determine the values of 2 not at the same temperature, 
but at the same pressure, the conditions under which the comparison is made will 
be more similar, and the resulting values may be expected to differ much less. In 
the calculation of the vapour pressures of a number of substances for each degree 
between certain limits of pressure, it became evident that at any given pressure the 
rate of increase was generally, though not always, greater for the volatile sub- 
stances than for the less volatile. 
A closer study of the matter led to the following generalisations :— 
1. The products of the rate of increase of pressure per unit difference of tem- 
perature into the absolute temperature = T are approximately the same for all 
stable substances at the same pressure, but the differences are real and are not due 
to errors of experiment or calculation. 
2. The rate of increase of this value a T with rise of pressure is the same for 
all stable bodies, at any rate for pressures between 150 and 2,000 mms., while for 
alcohol and water it is the same for all pressures between 100 and 20,000 mms. 
3. In the case of certain substances nearly related to each other, such as bromo- 
and chlorobenzene, or ethyl bromide and chloride, the ratio of the absolute tempera- 
tures of the related bodies at any given pressure is a constant. 
Complete paper to be found in ‘ Phil. Mag.,’ December 1885. 
5. Report on Optical Theories. By R. T. Guazesroox, M.A., F.B.S. 
See Reports, p. 157. 
6. On a Point in the Theory of Double Refraction. 
By R. T. Grazesroox, M.A., F.R.S. 
The author suggested that the theory of double refraction given by Lord Ray- 
leigh, in which the ether is supposed to have an effective density different in 
different directions, might be modified so as to agree with Fresnel’s theory if it be 
not necessary to assume that the ether offers an infinite resistance to compression, 
but only that as compared with its rigidity, its incompressibility is very great, and 
f 1885. 30 
