applied to the study of some Metamorphic Rocks. 1 73 



tity of water which it absorbs is more than 115 per cent. If 

 left exposed to the air for some time, the siliceous skeleton loses 

 the greater part of the water, but retains a mean quantity of 

 about 640 per cent., which corresponds very nearly with the 

 formula 3 Si, 0^, HO given by Beudant for an opake white 

 opal from Castellamonte. 



If it be immersed in a solution of ammoniacal sulphate of 

 copper, and afterwards exposed to the open air, it retains a por- 

 tion of the copper salt, even though subjected to repeated wash- 

 ings, and in the moist state it presents the appearance of certain 

 varieties of silicate of copper (copper hydrophane) ; in sulphuric 

 acid the substance becomes hyaline, and retains a portion of 

 the acid with great tenacity even after repeated washings. A 

 solution of caustic potash dissolves the skeleton with great faci- 

 lity even after ignition. 



Although it seems difficult to ascribe to this siliceous matter 

 a capability of forming definite compounds, yet the facts just men- 

 tioned are not less remarkable. The molecular condition in 

 which the silica exists in such alumino-magnesian compounds as 

 I have described, and the action which it exerts on a great 

 number of substances, would appear to indicate a point of con- 

 nexion between chemical phsenomena, strictly so called, and co- 

 hesive forces*. 



As an illustration of the decomposition and subsequent recon- 

 struction of rocks, I may refer to a pseudomorphite of quartz- 

 rock, in all probability derived from magnesite, and obtained 

 from nearly the same locality as the former mineral, which, 



* Sir J. Herschel, in his Introductory Address as President of the 

 Chemical Section of the British Association at the Leeds Meeting, 1858, 

 made some observations on the relation between capillarity and chemical 

 affinity, which appear to me to have a striking bearing upon the above 

 experiments. I cannot avoid quoting the following passage from that 

 discourse : — 



" There is another class of pha;nomena which, though usually considered 

 as belonging peculiarly to the domain of general physics, and so out of our 

 department, seems to me to want some attention in a chemical point of 

 view. It is that of cajnllary attraction. The coefficient of capillarity 

 differs very remarkably in different liquids, and no doubt also in their 

 contact with different solids, — a fact which can hardly be separated from 

 the idea of some community of nature between the capillary force and 

 those of elective attraction. I hardly dare to hint at the existence of some 

 slight misgiving I liave always felt as to the validity of the received sta- 

 tical theory of cajiillary action, which carries with it the a\ithority of such 

 iianits as tliose of Laplace and Poissou. Any discussion of this ])oint 

 would be matter for another section of tliis Association ; and if I here 

 touch uj)on it, it is only to observe that my impression of the requisiteness 

 of a force so far allied to chemical iiffiiiity as to he capable of saturation , 

 rests on other grounds besides tliat of the mere diversity of action above 

 alluded to." 



