132 
Dr. Blag den’s Experiments on the 
cularly on account of their very perfect and uniform tranfpa- 
rency. In this laft cafe, perhaps, another circumftance might 
contribute to die eafy cooling, that the acids, by combining 
with the water, feemed to expel the air it contained more per- 
fectly than moft other fubftances, as appeared from the innu- 
merable fmall bubbles that were almoft immediately formed. 
Want of tranfparency, however, is only one among feverai 
caufes which impair the property water naturally pofTefles, of 
bearing to be cooled many degrees below its freezing point. 
M. Mairan, in his elaborate Treadle upon Ice, having occa- 
lion to examine this fubjeft, was led by his experiments to 
conclude, that the cooling of water below its freezing point 
depends upon reft, and that agitation is the general caufe by 
which it is brought to fhoot into ice. In this opinion he has 
been almoft implicitly followed by all the writers I have fee n, 
excepting only Profeflor Wilcke, of Stockholm *. To bring 
it to the teft of experiment, I fet in the frigorific mixture fome 
diddled water, which by boiling had been rendered capable of 
fuftaining a cold of 21° before it froze. When this water was 
cooled to 22 0 , I agitated it, by moving the tumbler, by fhak- 
ing a quill in it, and by blowing on it fo as to ruffle the fur- 
face ; but it fupported all thefe trials without congealing, and 
did not fhoot till a minute or two afterwards, when by conti- 
nuance in the frigorific mixture it was cooled down to 21 0 . 
In other experiments, however, all the above-mentioned kinds 
of agitation made fimilar water inftantly congeal, even when 
not cooled fo low by feverai degrees. The congelation, there- 
fore, muff: in thefe cafes have depended on fome further cir- 
cumftance than the mere want of red. One that I fulpe&ed 
is a fort of tremulation, rather agitating fmall portions of the 
* Kongl. Vetenfk. Acad. Handlingar, Vol. XXX, p. 103. 105. 
7 
water 
