34 Notices of Memoirs — Gen. McMahon — On Serpentine. 



molecules of which crystalline bodies are composed are compacted 

 tightly together, like the cells of a honeycomb, without interspaces 

 between them ; and he referred to some experiments by Professor 

 Heddle on the absorption of water by granites and greenstones, and 

 to his own observations on the practical porosity of some basic 

 igneous rocks. Allusion was also made to the fact that the applica- 

 tion of the undulatory theory to the optical phenomena exhibited by 

 transparent bodies involved the assumption of molecular interspaces. 



General McMahon then alluded to the law that governs capillary 

 flow, and mentioned that pressure gauges set up in the Severn 

 tunnel 190 feet from the surface, showed that the actual pressure 

 of the water in the rocks at the tunnel exactly corresponded to the 

 calculated pressure, being about 801bs. per square inch ; a fact that 

 proved that the water permeating the rocks acted in one unbroken 

 head. He then alluded to the experiments of Poiseulle, who had 

 shown that capillary flow was increased by heat ; water at 45° 

 Centigrade flowing through capillaries three times faster than water 

 at zero Centigrade. 



The capacity of water to hold carbon dioxide and oxygen in 

 solution was next alluded to. Water at 60° is capable of taking up 

 rather more than its own volume of carbon dioxide, and meteoric 

 water contains two cubic inches of oxygen and one cubic inch of 

 carbon dioxide per gallon. As rain-water passes downwards into 

 the earth the percentage of oxygen is reduced and that of carbon 

 dioxide increased. He explained how this fact, as pointed out by 

 Prof. Heddle in a paper read before the Royal Society, Edinburgh, 

 accounted for the ferrous oxide in olivine being removed as carbon- 

 ate, when rocks were acted on at some depth, but converted into 

 magnetite, or ferric oxide, when subjected to aqueous agencies nearer 

 the surface. 



Carbonated water is capable of decomposing the silicate of 

 magnesia, and of carrying off some of the magnesia in the form 

 of carbonate, as proved experimentally by Bischof. Profs. W. E. 

 and E. E. Eogers further showed that digestion in simple water for 

 three days was sufficient to remove an appreciable amount of 

 magnesia from such minerals as hornblende. General McMahon 

 stated that he obtained a similar result by the digestion of powdered 

 olivine in distilled water heated to about 100° F. 



The formula for olivine is 2MgO, 2FeO, Si0 2 , and it was 

 explained in detail how the removal of the ferrous oxide, a portion of 

 the magnesium silicate, and the absorption of water converted olivine 

 into serpentine. The formula for the latter is 3MgO, 2Si0 2 , 2H 2 0. 



Taking enstatite and malacolite as types of the rhombic and 

 monoclinic pyroxenes, and treniolite as that of the amphiboles, he 

 explained how the lime was removed and the percentage of 

 magnesia was increased. Whilst olivine, the predominant mineral, 

 was parting with some of its magnesia, the silica set free in the 

 pyroxene by the decomposition of the silicate of lime combined with 

 this surplus magnesia on its exit from the olivine and a gradual 

 conversion of pyroxene into serpentine was the result. 



