the Production of artificial Cold. 
*37 
POSTSCRIPT. 
In the course of my former papers on the subject of cold, 
I have had occasion to make, incidentally, some remarks on 
the power of water, under certain circumstances, in resisting an 
extraordinary degree of cold without freezing ; likewise on 
the particular kind of agitation which induces water, cooled 
below its freezing point, to crystallize or become ice.* As 
these are subjects which have likewise engaged the attention 
of others, I shall take the liberty of barely mentioning a fact, 
having relation to those points, which has lately occurred to me. 
It is a remarkable circumstance respecting artificial freez- 
ing, that the ice thus procured in the usual way, (viz. by im- 
mersing the water to be frozen, in a convenient vessel, in 
a frigorific mixture,) will always be more or less opake, never 
transparent: this I had constantly remarked, without much 
attending to it ; however, having in the course of last summer 
been induced to try the effect of an ice-speculum for pro- 
ducing heat, it became necessary that the ice, which in this 
instance was substituted for glass, should be perfectly trans- 
parent. After varying the process in every possible way I could 
think of, by immersing the water to be frozen, without effect, I 
at last succeeded completely, by forming a coating of ice, of 
sufficient thickness, on the outside of a vessel containing the 
frigorific mixture ; the bottom of this vessel, which was made 
concave for this particular purpose, being immersed for a suffi- 
cient length of time in a shallow pan of water. 
* Phil. Trans, for 1788, page 40*. 
T 
MDCCCf. 
