W. Skey. — Evolution of Heat during Hydration of Clay slate. 385 



These results tend to show that, as a general rule, any so-termed hygros- 

 copic substances, when deprived of even the smallest portion of their water, and 

 then allowed to regain this, generate heat in doing so. It will at once appear 

 that, if in place of submitting water to these dried substances we submit 

 aqueous vapour, the evolution of heat would be much greater, and in fact I 

 find that these substances desiccated and exposed to common air rise in 

 temperature very notably. 



It only remains now to consider how much of this elevation of temperature 

 is due to mechanical, and how much to chemical agency. 



It is obvious a portion of this is due to friction, occasioned by the rapid 

 inrush of water to the pores of these substances. I believe the calorific efiect 

 of such inrushes has not yet been measured, if indeed noticed, before, and in all 

 likelihood they will be deemed so small as to be barely perceptible, or at most 

 not at all necessary to take into account in determining the origin of the 

 increase of temperature instanced ; but as I felt anxious to get the approximate 

 calorific result due to chemical agency, I have made an attempt to effect this 

 by comparative tests. 



In this attempt I substituted other liquids for water, liquids which, 

 experimentally, I found had no action upon the solids used, or only to a very 

 minute extent. 



My results were as follows : — 



I. Clay-slate mixed with water raised a thermometer placed in the mixture 

 2° F. above the temperature of these substances, at the time of mixing 

 them. A portion of the same sample of clay-slate, and in quantity as 

 before, mixed with pure kerosene in same volume as that of the water 

 used, only raised the thermometer 1J° F. 

 II. Dried brown coal, similarly treated, gave an elevation of temperature 

 equal to 4° F. with water, and only 2° F. with oil. The same conditions 

 were observed as to quantities and volumes as in the first experiment. 



As the kerosene used would have a less specific heat than water, the amount 

 of heat due to friction in the first experiment would not be so much as 1J°, 

 and in the second experiment not so much as 2°, leaving a balance of heat 

 equal to something more than |° and 2° F. for the slate and coal respectively, 

 which balance is, I conclude, due to chemical action ; and as I believe kerosene 

 is more diffusive than water, and so would rush these substances with greater 

 rapidity than water would, thus producing more friction, the balance of heat 

 found is, perhaps on this account, again less than the actual amount due to 

 chemical action. 



Keturning now to the subject I started with, the supposed direct hydration 

 of clay-slate by water, and bringing the results just stated to bear upon this 

 question, it certainly appears that this substance evolves heat when mixed 



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