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(2) The occurrence of serpentine in beds interstratified with 
sedimentary rocks. 
(3) The frequent association of serpentine with dolomite and 
limestone. In these cases in which dolomite contains the requisite 
amount of silica, all the magnesia would be converted into 
serpentine, and it would be found associated with limestone only. 
(4) Its association with schists, which were probably altered from 
the clays and sandstones interstratified with the dolomite, by the 
same agencies as those which converted the dolomite into serpentine. 
In conclusion, we may state that we are engaged in making 
experiments upon the direct conversion of the Campsie limestone 
into serpentine, and upon the influence exercised by one carbonate 
upon the decomposing point of another, with which it is mixed. 
Monday , 3 rd April 1882. 
Professor BALFOUR, M.D., Vice-President, 
in the Chair. 
The following Communications- were read : — 
1. On the Figures of Equilibrium of a Rotating Mass of Fluid. 
By Sir William Thomson. 
(a) The oblate ellipsoid of revolution is proved in Thomson and 
Tait’s Natural Philosophy (first edition, § 776, and the Table of 
§ 772) to be stable, if the condition of being an ellipsoid of 
revolution be imposed. It is obviously not stable for very great 
eccentricities without this double condition of being both a figure 
of revolution and ellipsoidal. 
(b) If the condition of being a figure of revolution is imposed, 
without the condition of being an ellipsoid, there is, for large 
enough moment of momentum, an annular figure of equilibrium 
which is stable, and an ellipsoidal figure wdiich is unstable. It is 
probable, that for moment of momentum greater than one definite 
limit and less than another, there is just one annular figure of 
equilibrium, consisting of a single ring. 
(c) For sufficiently large moment of momentum it is certain 
