3 1 2 'M r . c A v e n dt s if s Obfervatldns' 
If it was not for this generation of heat by the aft of freez- 
ing, whenever a veil'd of water, expofed to the cold, was ar- 
rived at the freezing point, and Began to freeze, the whole would 
iiiftantly be turned into folk! ice ; for as the new formed ice is not 
* i - 
fenfbly colder than water beginning to freeze, it follows, that 
as foon as all the water in the v'effel was cooled to that point, 
the leaf; addition of cold would convert the whole into ice ; 
whereas it is well known, that though the whole veil'd of wa- 
ter is cooled to, or even below, the freezing point, there is a ' 
long interval of time between its beginning to freeze and being 1 
intirely frozen, during all which time it does not grow at all 
colder. 
• * t m 
In like manner, it is the cold generated by the melting of 
ice which is the canfe of the long time required to thaw ice or 
fnow. It is this alfo which is the caufe of the cold produced 
by freezing mixtures ; for no cold is produced by mixing fnow 
with any fubfiance, unlefs part of the fnow is difolved. 
I formerly found, by adding fnow to warm water, and fur- 
ring it about till all was melted, that the water was as much 
cooled as it wbuld have been by the addition of the fame quan- 
tity of water, rather more than 150° colder than the fnow ; oiy 
in other words, fomewhat more than 150° of cold are gene- 
rated by the thawing of fnow ; and there is great reafon to 
think, that juft as much heat is produced by the freezing of 
water. The cold generated was exactly the fame whether I 
ufed ice or fnow *. 
I have - 
I am informed, that Dr. black explains the abovementioned phenomena in 
the fame manner ; only, inliead of ufing the expreffion, heat is generated or pro- 
duced, he fays, latent heat is evolved or fet free ; but as this expreffion relates to 
an hypothecs depending on the fuppofition, that the heat of bodies is owing to 
their 
