VOL. LXXVllI.] PHILOSOPHICAL TRANSACTIONS. 413 



with regard to the others, I think the variation was no greater than usually 

 takes place with different portions of common water. Scarcely any, perhaps 

 none, of the above-mentioned points were absolutely the lowest to which the 

 solutions or mixtures could have been reduced, if the experiment had been con- 

 ducted still more slowly and cautiously. But however much they might all pos- 

 sibly have borne to be cooled, a great difference occurred among them in the ease 

 with which the operation succeeded. 



Want of transparency liovvever is only one among several causes which im- 

 pair the property water naturally possesses, of bearing to be cooled many degrees 

 below its freezing point. M. Mairan, in his elaborate treatise on ice, having 

 occasion to examine this subject, was led by his experiments to conclude, that 

 the cooling of water below its freezing point depends on rest, and that agitation 

 is the general cause by which it is brought to shoot into ice. In this opinion 

 he has been almost implicitly followed by all the writers I have seen, excepting 

 only Professor Wilcke, of Stockholm. To bring it to the test of experiment, 

 I set in the frigorific mixture some distilled water, which by boiling had been 

 rendered capable of sustaining a cold of 21° before it froze. When this water 

 was cooled to 22° I agitated it by moving the tumbler, by shaking a quill in it, 

 and by blowing on ii so as to ruffle the surface ; but it supported all these trials 

 without congealing, and did not shoot till a minute or 2 afterwards, when by 

 continuance in the frigorific mixture, it was cooled down to 21°. In other ex- 

 periments however all the above-mentioned kinds of agitation made similar water 

 instantly congeal, even when not cooled so low by several degrees. The conge- 

 lation therefore, must in these cases have depended on some further circum- 

 stance than the mere want of rest. One that I suspected is a sort of tremula- 

 tion, rather agitating small portions of the water separately, than moving the 

 whole together. I have found, that striking the bottom of the tumbler against 

 a board would produce instant congelation, when stirring the water, or shaking 

 the tumbler in the hand, would have no effect. In like manner when, in stir- 

 ring the cooled water, the quill or stick of glass, employed for that purpose, 

 strikes against the side or bottom of the tumbler, the water, which had resisted 

 the general stirring, is often by this percussion made to freeze. The same effect 

 is produced, and with less uncertainty, if the quill or stick of glass be rubbed, 

 and as it were ground, against the side of the tumbler. But of all such me- 

 thods of bringing on the congelation, that which I have found to fail the 

 seldomest, is to rub a bit of wax against the side of the tumbler under the 

 water ; a particular roughness in the motion is felt, with some sound, approach- 

 ing to a musical tremulation, and a crust of ice is immediately perceived under 

 the wax on the glass. This effect of the wax I take to be mechanical, depending 

 on its particular state of consistence. Wood acts in the same manner, though 



