the Point of Congelation. 295 
And fo much a&ually is produced before the whole liquefa&ion 
is effected ; and yet a mixture of thefe two fubftances will fink 
the thermometer no lower than to 4 0 of Fahrenheit’s fcale. 
The conlideration of this apparent difficulty has led to the 
fuppofition, that a certain quantity of fire is contained in the 
cryftals of the fait, which being difengaged in the folution 
keeps up the mixture to a certain temperature*. But I con- 
ceive, that the phenomenon depends (imply on the gradual 
liquefaction of the ingredients, a neceffary confequence of the 
cold produced. A faturated folution of fal ammoniac freezes 
itfelf at 4 0 ; therefore, when the mixture is reduced, by the 
liquefaction of the ingredients, to that temperature, no more 
of them can melt, becaufe any addition of cold would freeze 
what is already melted ; and if the mixture, under luch cir- 
cumftances, were placed in an atmofphere of its own tempera- 
ture, the ingredients would remain for ever in that fame date, 
without any further liquefaction. But in an atmofphere 
warmer than 4 0 , they continue to melt, more or lefs fiovvly, as the 
heat which is gradually communicated furnilhes what is necef- 
fary to become latent. This communicated fenfible heat being 
immediately converted into latent, the mixture will always be 
kept down to the fame temperature as long as there is a fuffi- 
cient mafs of unmelted materials ; and it can fink no lower, 
becaufe then the liquefaCtion would be (lopped ; confequently 
fuch mixtures mud prelerve, as they have been found to do, a 
pretty uniform temperature, fo as to have been formerly ufed 
for graduating thermometers. And the whole cold produced, 
or, to fpeak properly, the whole of the heat made to difap- 
pear, I prefume to be ultimately equal to the full quantity of 
latent heat belonging to the diffolved ice and fait. 
* D e Luc’s Idces fur la Mcteorologie, Tom. I. p. 201- 
According 
