25 
Steam Engine. 
to 32®, and will there remain stationary a considerable 
time before it congeals. Yet while thus stationary, it 
cannot be doubted, that the pure water is yielding caloric 
to the atmosphere, equally with the saline solution ; for it 
is impossible that a warmei* body can be surrounded by a 
cooler one, without imparting caloric to the latter. The 
reason of this equable temperature is well explained by 
Dr. Crawford. Water, he observes, 
during freezing, is acted upon by two opposite powers : 
it is deprived of caloric by exposure to a medium, whose 
temperature is below 32° ; and it is supplied with calo-* 
ric, by the evolution of that principle from itself, viz. of 
that portion which constituted its fluidity. As these pow- 
ers are exactly equal, the temperature of the water must 
remain unchanged, till the caloric of fluidity is all evolv- 
ed. 
3. The evolution of caloric, during the congelation of 
water, is well illustrated by the following experiment of 
Dr. Crawford : — Into a round tin vessel put a pound of 
powdered ice ; surround this by a mixture of snow and 
salt in a larger vessel ; and stir the ice in tlie inner one, 
till its temperature is reduced to +4° of Fahrenheit. To 
the ice thus cooled, add a pound of water at 32®. One 
5th of this will be frozen ; and the temperature of the ice 
will rise from 4® to 32®. In this instance, the caloric, 
evolved by the congelation of one 5th of a pound of water, 
raises the temperature of a pound of ice 28®. 
4, If we dissolve sulphate of soda in water, in the pro- 
portion of one part to five, and surround the solution by 
a freezing mixture, it cools gradually down to 31®. The 
salt, at this point, begins to be deposited, and stops the 
cooling entirely. This evolution of caloric, during the 
separation of a salt, is exactly the reverse of what happens 
during its solution, (Blagden, Philosophical Transac- 
tions, Ixxviii. 290.) 
VoLIL ‘ D 
