414 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



with less certainty ; so does a quill, and also glass ; but the latter, being very- 

 hard, produces the effect with least certainty. It is a mechanical action on the 

 water in contact with the rubbing substance and the glass : for if the outside of 

 the tumbler, or any part of the inside above the water, be rubbed, even if it be 

 wet so as to communicate a similar feeling of tremulation, yet still the congela- 

 tion is not produced. All these modes of bringing on the congelation succeed 

 best, as might be expected, in proportion as the water is more cooled below the 

 freezing point. Unless the cooling amount to 4 or 5°, the friction with wax is 

 often in vain. 



From the above-mentioned facts it appears, thatM. Mairan's position, though 

 not destitute of foundation, was enounced by him too generally, and without 

 sufficient precision. It is the natural property of water to bear to be cooled a 

 certain number of degrees without freezing ; rest favours this property nega- 

 tively, by giving it no interruption ; but most kinds of agitation interfere with 

 its operation to a greater or less degree, and some perhaps would prevent it alto- 

 gether ; while others affect it so little, as not to superinduce the congelation, 

 even when the cooling is brought within 1° of the greatest that the water will 

 bear. 



Whatever be the effect of agitation, there is another cause which much more 

 powerfully hastens the congelation of water. It has been long known, that 

 when water is cooled below its freezing point, the contact of the least particle ot 

 ice will instantly make it congeal, the glacial crystals shooting all through the 

 liquor, from the spot where the ice touches it, till the whole comes up to the 

 freezing point. Few experiments of the minute kind afford a more striking 

 spectacle than this, especially when the water has been cooled nearly as much as 

 possible below the freezing point ; both from the beautiful manner in which 

 the crystals shoot through it, and the rapidity with which the mercury in the 

 thermometer immersed in it runs up through a space of 10 or 1 1°, stopping and 

 fixing always at 32 in pure water. If from any circumstance however, as a less 

 cooling, or the addition of a salt, the shooting of the ice proceed more slowly, 

 the thermometer will often remain below the freezing point even after there is 

 much ice in the liquor ; and does not rise.rapidly, or to its due height, till some 

 of the ice is formed close to its bulb ; which exemplifies the evolution of the 

 latent heat from the very particles that congeal. 



Many of the circumstances attending the greater or less cooling of water 

 below its freezing point depend on this principle. In a calm day, when the 

 temperature of the air was about 20*^, I exposed 2 vessels with distilled water to 

 the cold ; one of them was slightly covered with paper, the other was left open : 

 the former bore to be cooled many degrees below the freezing point, while a crust 

 of ice always formed on the surface of the other before the thermometer im- 



