VOL. LXXVIIl.] PHILOSOPHICAL TRANSACTIONS. 41/ 



when water is cooled down to 32°, it is already sensibly expanded ; and if the 

 congelation does not take place here, this ejjpansion augments, in proportion as 

 the water is further cooled *. The expansion therefore being so evidently an ap- 

 proach to freezing, may be considered as art indication that the polarity already 

 prevails so far as to draw the particles somewhat out of the situation they naturally 

 assume in the higher temperatures. And it is conceivable, that if this operation 

 go on very quick, and the consequent change of position in the particles be 

 made with some degree of velocity, they may acquire a small momentum of 

 motion, enabling them to overcome a resistance which would otherwise prevent 

 their junction. 



To assist the conception, I have here reasoned on the particles of water as 

 solid^ and of a determinate shape. But it seems most probable, that the par- 

 ticles of matter in general are nothing more than centres to certain attractive and 

 repulsive powers; on which hypothesis it may be understood, that if 2 or more 

 of these central points are brought much within the limits of their respective at- 

 tractions and repulsions, these powers will no longer be equal at equal distances 

 from their common centre. Now such a combination of central points may be 

 considered as 1 particle of any particular matter; and the unequal distances from 

 the common centre at which the attractions and repulsions are equal will define 

 what may be called the shape of that particle. And if, at equal distances, the 

 attraction or repulsion is much greater at one point than at another, that will 

 constitute a polarity. 



The greatest cold I have been able to make water acquire without freezing, is 

 near 1 2° of Fahrenheit's scale below its common freezing point. Some distilled 

 'water was boiled about a quarter of an hour in a tin cup, and placed in the same 

 vessel, while still warm, in the frigorific mixture. The mixture was made to act 

 very slowly, so that the operation continued more than an hour. When the im- 

 mersed thermometer had sunk to 20°^-, the water was still fluid: I then shook 

 it considerably, but no ice formed. After waiting some time, and finding the 

 therm.ometer would sink no lower, because by the length of the process the 

 snow of the mixture was almost consumed, I added some fresh materials, which 

 could not be done without shaking the tin cup. Still however the water did not 

 freeze instantly, though it shot as soon after as it can be supposed to have felt 

 the influence of the new frigorific mixture. When this water was cooled to 24°, 

 I tried the temperature of the air near its surface, and found it 34° or 35°, the 



* In experiments where tlie water has cooled much below its freezing point, I have seen the ex- 

 pansion so great as to bear a considerable proportion to the whole expansion produced by freezing, 

 which last I believe is more than -^ of the volume of the water. It seemed as if the expansion pro- 

 ceeded in an increasing ratio, being much greater on the last degrees of cooling than it was on the 

 first. The difficulty of procuring a proper apparatus for these experiments has hitherto prevented 

 me firom ascertaining the quantities with precision. 



VOL. XVI. 3 H 



