386 Transactions, — Chemistry. 



Trith water, which heat is the result of chemical action, and the only agency to 

 which I can attribute this chemical action is hydration. 



Thus the statement hazarded as to the direct hydration of clay -slate by 

 water receives support upon the ground I have just travereed. 



Applying now the facts just elicited in a geneml manner, it appears — 



(].) That in the disintegration of rocks or soils heat is evolved. 



(2.) That the differences in temperature sometimes observed between 

 contiguous strata may be due, wholly or partly, to this cause. 



(3.) That our native anhydrous coals hydrate upon their surfaces when 

 exposed to water or aqueous vapour. 



(4.) That hygroscopic water is chemically-combined water. 



(5.) That the quantity of water present in certain rocks or minerals may, 

 when known, frequently indicate the highest temperature to which 

 they have been subjected. 



(6.) That vegetable matter (leaves, twigs, etc.) generally developes heat 

 by hydration ; also by friction, when the temperature of the air 

 suiTOunding it is lowered. 



In regard to these statements it requires, in the case of (3), gravimetrical 

 experiments to support it, which I shall presently endeavour to obtain, and if 

 they should prove it a correct one, that is, that anhydrous coal can hydrate, 

 and to any notable extent, it will certainly appear that this substance has 

 been formed at a somewhat elevated temperature, perhaps approaching to 

 nearly 100^ C. 



While upon the subject of the formation of coal, I would just like to 

 observe here, that I cannot avoid thinking the effects of pressure in 

 consolidating this, and indeed other minerals, also rocks, are considered much 

 greater than they really have been, and this because it seems that these 

 substances will generally, if not always, be charged with water, oil, or gas, 

 and, if this is so, I conceive the consolidating action of pressure would be very 

 greatly mitigated, and would be in some proportion to its actual volumetrical 

 effect upon the liquid receiving it. I cannot see how particles suspended in, or 

 throughly soaked with a liquid, can be made to approach each other by 

 pressure, except by allowing the liquid to escape, and it does not appear, in 

 the case of rocks, etc., at some depth, that there can be any such way of escape, 

 at least a sufficiently ready one, for the liquids or ga.ses lotlged in their poi*es. 



In refercncG to the statement (4) that the hygroscoj^ic water of substances 

 retaining it is combined water, this appears certain from what has been 

 desciibed, and also from other considerations. Thus, to take an analogous 

 case, the salt chloride of silver forms a definite crystallizable chemical 

 compound with ammonia, but, though acknowledged as such, it can only be 

 preserved in an atmosphere of ammonia, and if this gas is taken away as 



