47 



stances of the phenomenon perfectly well." " I formerly found, by adding 

 snow to warm water, & stirring it about till all was melted, that the water 

 was as much cooled as it would have been by the addition of the same 

 quantity of water, rather more than 150° colder than the snow, or in other 

 words, somewhat more than 150° of cold are generated by the thawing of 

 snow, & there is great reason to think that just as much heat is produced 

 by the freezing of water. The cold generated was exactly the same, whe- 

 ther I used ice or snow. (Note.) I am informed that Dr. Black explains 

 the above-mentioned phenomena in the same manner, only instead of using 

 the expression 'heat is generated or produced,' he says, 'latent heat is 

 evolved or set free'; but as tins expression relates to an hypothesis depending 

 on the supposition that the heat of bodies is owing to their containing more 

 or less of a substance called the matter of heat ; 8f as I think Sir Isaac New- 

 ton's opinion, ' that heat consists in the internal motion of the particles of 

 bodies,' much the most probable, I chose to use the expression 'heat is gene- 

 rated.' Mr. Wilcke also, in the Transactions of the Stockholm Academy 

 of Sciences, explains the phenomena in the same way, & makes use of an 

 hypothesis nearly similar to that of Dr. Black. Dr. Black, as I have 

 been informed, makes the cold produced by the thawing of snow 140° ; 

 Mr. Wilcke, 130°." 



He then goes on to mention briefly that he had formerly kept a thermo- 

 meter in melted tin and lead, &c. And this is all he has said of experiments, 

 made 19 years before, the notes of which fill 120 pages, 8vo., and the paper of 

 results and deductions from them, 50 pages, 4to. : the following extracts will 

 give a general idea of the contents of the latter, and of a fragment which seems 

 to have been written still earlier.] 



FRAGMENT ON HEAT. 



Hypothesis. 



All bodies, in changing from a solid state to a fluid state, or from a non- 

 elastic state to the state of an elastic fluid, produce cold, & by the contrary 

 change they produce heat. 



The principal cases in which bodies are changed from a non-elastic to an 

 elastic state, are the evaporation of liquors, & the separation of fixed air from 

 alcaline substances. 



Before we consider how far this hypothesis agrees with experiment, it may 

 be proper to premise, that most bodies which have any considerable affinity to 

 each other, generate heat in mixing. This is well known to be the case in ma- 

 ny instances : such as mixing oil of vitriol, & the 2 other mineral acids, spirits 

 of wine, quick lime, & dry pearl ashes, with water, & many other instances, 

 though in some, such as the solution of salts in water, there seems to be cold 

 generated, & there seems to be cold produced by the mixture of substances 

 which have an affinity to each other, as in the solution of salts in water, but 

 this will be shown to be owing to another cause. I very much question, in- 

 deed, whether there is any real instance of cold being produced by the mix- 

 ture of 2 bodies which have no affinity to each other. 



