1886.] the revolution of Limagons and Cardioids about their axes. 19 
K, (0) * & J, (0)d0 
- ak (r) af a? J, (da) da - 
7 pay 
=* (K,+ 20K); 
. F(a) =F (HGS ONE atts snk Mau (40); 
whence the attractions are determined. 
(2) An attempt to explain certain geological phenomena by the 
application to a liquid substratum of Henry’s law of the absorption 
of gases. By Rev. O. FisHer, M.A. 
HEnry’s law of the absorption of gases by liquids asserts, 
that the volume of the gas which can be held in solution by the 
liquid is the same, whatever be the pressure. But since a given 
volume of gas contains, by Boyle’s law, a mass proportional to the 
pressure, it follows, that the mass of a gas that can be held in 
solution by a given quantity of liquid at a constant temperature 
varies as the pressure. 
Suppose that a liquid substratum exists beneath the earth’s 
crust, and that it consists of fused rock, holding gas (chiefly water 
above its critical temperature) in solution. It is believed by the 
writer to be this water which is given off during volcanic erup- 
tions*. If such be the constitution of the substratum, it is obvious 
that, not only will the reactions between this and the crust largely 
depend upon it, but also the tidal effects of the moon and sun 
upon the earth be modified. 
Let m be the volume of gas held in solution in unit volume 
of the liquid, yp the mass of the same under the pressure p; 
where m and vy are two constants, m depending upon the solubility 
of the particular gas in the particular liquid at the given tem- 
perature, and y depending upon the compressibility of the gas at 
that temperature. 
If the analogy of water dissolving different gases at ordinary 
temperatures can be taken as a guide, it would seem that m will 
vary greatly for different gases, as appears from Henry’s experi- 
ments. In the case of carbonic acid and water he found m to be 
about unity. In the case of sulphuretted hydrogen and water, 
m was about 086+. 
* Prof. Prestwich refers the presence of steam in volcanic eruptions to the lava 
encountering water, derived from meteoric or marine sources, during its rise in the 
vent. Proc. Roy. Soc. Apr. 16, 1885. 
+ Phil. Trans. Roy. Soc. 1808. 
2—2 
