i ;6 Dr. Blag den’s Experiments on the 
furface of the water be in no wife defended from the cold air 
of the laboratory. Oil fpread over the furface of water has 
been found to prevent it from freezing, when other water 
fimilarly expofed has had a cruft of ice formed upon it. This 
1 afcribe entirely to the prevention of frozen particles from 
coming in contact: with the water : for in experiments with fri- 
gorific mixtures, in a room of moderate temperature, I do not 
find that oil on the furface has any fenfible effect in enabling 
water to fupport more cold, unlefs, indeed, where the 'Opera- 
tion is otherwiie too much precipitated. Alfo a crack in the 
tumbler containing the water prevents it from cooling below 
the point of congelation, a thin film of ice infinuating itfelf 
through the crack into contact with the water. And often, in 
experiments with frigorific mixtures, the congelation is brought 
on by railing the immerfed thermometer a little out of the 
water, and lowering it down again ; fome of the adhering 
water having frozen upon its ftem. 
Several other circumftances, though not fo diftinclly afcer- 
tained as the preceding, appear to facilitate the congelation of 
cooled water. For inftance, in experiments with frigorific mix- 
tures, if the cold be very intenfe, the water freezes almoft 
immediately round the fides of the veffel, as if fomething 
• depended on too fudden a change of temperature. Accord- 
ingly, the only way of infuring the greateft degree of cold in 
water without freezing, is to coo] it in a very gradual manner, 
keeping the cold of the frigorific mixture regularly only two 
or three degrees below that of the water. Sudden cooling, there- 
fore, may be confidered as one of the caufes which haften conge- 
lation. No doubt this will fometimes depend on luch a cold as 
water cannot refift without freezing, being propagated through 
the glais to the neareft part of the water, quicker than it can 
be 
