2 D. Forbes — Contraction of Igneous Rocks. 



various furnace slags, both acid and basic, and still more acid sili- 

 cates like glass. 



The results of this inquiry fully satisfied me that only comparative 

 and but roughly approximative data, as to the actual amount of con- 

 traction undergone, could be obtained by such experiments as those 

 of Bischof and myself, since, when operating upon so small a scale, 

 it was quite impossible to avoid various sources of error, or to obtain 

 the silicate, after cooling, so free from vacuities, or, in other words, 

 so compact, as when working on an infinitely larger scale, as is the 

 case in nature. For these reasons, therefore, I did not, as Bischof 

 appears to have done, attempt to deceive myself by reducing the 

 actual values obtamed in these experiments to a numerical expression 

 of the percentage amount of the contraction ; but, instead of putting 

 any great confidence in such figures, was content to point out that 

 the results of all these experiments were unanimous in indicating 

 that the actual amount of contraction experienced must be greatly 

 below that which, upon Bischof 's authority, seems to have been veiy 

 generally accepted. 



A summary of my experiments has been published in the Chemical 

 Neivs for October 23, 1868, and I now find, in a recent paper " On 

 the Elevation of Mountains by Lateral Pressure, its Cause, and the 

 Amoimt of it, with a Speculation on the Origin of Volcanic Action," 

 by the Eev. 0. Fisher, M.A., F.G.S., published in the Transactions 

 of the Cambridge Philosophical Society, vol. xi., part iii., that they 

 have been commented upon as follows : — 



" Mr. David Forbes, in a paper in which he combats the idea of 

 the solidity of the earth, and is concerned to reduce as much as 

 possible the amount of contraction of its materials, states that, in 

 the manufacture of artificial stone from the Eowley Rag, the specific 

 gravity of the Eag was 2-84, while that of the fused stone, after 

 having been cooled slowly and devitrified in red-hot sand moulds, 

 was the same (2-84). But the specific gravity of the glass before 

 devitrification was only 2-67. The volume being necessarily in 

 inverse proportion to the density, these numbers give a contraction 

 of 0-060 in passing from the glassy to the stony state. 



" Mr. Forbes himself made some experiments upon highly sili- 

 ceous slags, which he cast in iron moulds of 360 cubic inches, 

 and found the slags to contract to from 350 to 355 cubic inches. 

 These numbers give a contraction of from 0"014 to 0-028. In the 

 latter case a further contraction would probably accompany devitri- 

 fication, which, from the analogy of the Eowley Eag, would raise 

 the contraction to from 0.074 to 0.088. 



" The dimensions of the iron moulds above given seem to have 

 been taken when they were cold. If this were so, the above 

 numbers would be too small." 



As I have already given the reasons which led me to inquire 

 experimentally into the ratio of the contraction of igneous rocks, 

 and since the question, whether such contraction is as large as 

 Bischof estimates it, or as small as I found it, cannot have any 

 influence, pro or contra, on the views or arguments which I have ad- 



