4i6 PHILOSOPHICAL TBANSACTIONS. [aNNO 1788. 



buted to the rest of the water ; but I think the above-mentioned effect takes 

 place when no part of the fluid can be supposed to be many degrees below the 

 freezing point. 



It has been alleged, that metal in contact, either with the outside of the 

 vessel containing the water, or with the water itself, disposes it to freeze sooner 

 after it is cooled below 32°. Though on repeating this experiment I have found 

 it possible to cool water in a metal vessel manv degrees below its freezing point, 

 and even to touch it, when so cooled, with metal equally cold, without producing 

 congelation ; yet the metal certainly tends to hasten the freezing, and I believe 

 un the above-mentioned principle of too quick a change of temperature, occa- 

 sioned by its quality as a good conductor of heat. For the same reason it is 

 more difficult to cool wa.ter much below the freezing point in thin vessels, than 

 in those whose bottom and sides are of considerable thickness ; the latter trans- 

 mitting the heat more slowly, and allowing it thus to be diffused more equably. 



In cooling water below its freezing point by frigorific mixtures, it is of con^'* 

 sequence to keep the mixture some way below the upper edge of the water 

 within the tumbler, otherwise the congelation quickly begins at that place. This 

 very likely depends on the principle last mentioned, that the thin edge of water 

 rising up against the side of the glass, being more in contact with air than with 

 the general mass of water, does not so easily distribute its cold, and therefore 

 suffers a more rapid change of temperature by the action of the mixture. Hence 

 one of the most essential precautions for cooling water to the utmost without con- 

 gelation, is to perform the experiment in a warm room, that the air in contact 

 with the edges and surface of the water may prevent their sudden cooling. And 

 one of the most convenient vessels for the purpose is a round body terminating 

 in a neck, the body to be surrounded with the frigorific mixture, while the water 

 in the neck is kept above the freezing point. These are the principal facts with 

 which my experiments have furnished me relative to the cooling of water below 

 its point of congelation. I see no general circumstance that applies to them all. 



Sudden cooling may promote congelation simply by occasioning the water at the 

 bottom and sides of the vessel to acquire a greater degree of cold than the rest. 

 But perhaps it may have also another effect, admitting of a particular explanation. 

 Water in freezing undergoes a considerable expansion. This may be ascribed to 

 such a form of its particles, and position of their poles, as shall make them, 

 when touching and adhering by those poles alone, intercept very large interstices, 

 which may be considered as the pores of the ice. Various positions of the poles 

 and figures of the particles may be conceived, which should cause them to oc- 

 cupy more space, when touching in certain points only, than they filled when 

 lying near without any contact. But in whatever way the expansion is produced, 

 experiment hath shown that it begins some time before congelation; so that 



