424 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



If it was not for this generation of heat by the act of freezing, whenever a 

 vessel of water, exposed to the cold, was arrived at the freezing point, and 

 began to freeze, the whole would instantly be turned into solid ice ; for as the 

 new formed ice is not sensibly colder than water beginning to freeze, it follows, 

 that as soon as all the water in the vessel was cooled to that point, the least addi- 

 tion of cold would convert the whole into ice ; whereas it is well known, that 

 though the whole vessel of water is cooled to, or even below the freezing point, 

 there is a long interval of time between its beginning to freeze and being entirely 

 frozen, during all which time it does not grow at all colder. 



In like manner, it is the cold generated by the melting of ice which is the 

 cause of the long time required to thaw ice or snow. It is this also which is the 

 cause of the cold produced by freezing mixtures ; for no cold is produced by 

 mixing snow with any substance, unless part of the snow is dissolved. Mr. C. 

 formerly found, by adding snow to warm water, and 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 J 50° colder than the snow ; or, 

 in other words, somewhat more than 1 50° of cold are generated by the thawing 

 of snow ; and 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 whether he 

 used ice or snow.* He formerly kept a thermometer in melted tin and lead till 

 they became solid ; the thermometer remained perfectly stationary from the time 

 the metal began to harden round the sides of the pot, till it was entirely solid ; 

 but he could not perceive it to sink at all below that point, and rise up to it when 

 the metal began to harden. It is not unlikely however, that the great difference 

 of heat between the air and melted metal might prevent this effect from taking 

 place ; so that though he did not perceive it in those experiments, it is not un- 

 likely that those metals as well as' water and quicksilver, may bear being cooled a 

 little below the freezing or hardening point (for the hardening of melted metals 

 and freezing of water seems exactly the same process) without beginning to lose 

 their fluidity. 



Mr. Hutchins's first 5 experiments were made with the apparatus, and in the 



* I am informed, says Mr. C, that Dr. Black explains the abovementioned 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 this expression relates to an hypothesis depending on the supposi- 

 tion, that die heat of bodies is owing to their containing more or less of a substance called the matter 

 of heat ; and as I think Sir Isaac Newton'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 generated- 

 Mr. Wilke also, in the Transactions of die Stockholm Academy of Sciences, explains the phenomena 

 in the same way, and makes ue of an hypothesis nearly similar to diat of Dr. Black. Dr. Black, as 

 I have been informed, makes die cold produced by the diawing of snow 1-K) ; Mr. Wilke, 130°. — 

 Orig. 



