VOL. LXXVIIt.J PHILOSOPHICAL TRANSACTIONS. 415 



mersed in the middle of it came to the freezing point. This phenomenon, 

 which other observers have remarked without being able to account for it, ap- 

 pears clearly owing to frozen particles, which in frosty weather are almost always 

 floating about in the air, often perceptibly to the senses. They come most 

 commonly either from clouds passing over head, or from snow or hoar-frost 

 lying on the earth ; and when they touch the cooled surface of the water, in- 

 stantly make it freeze. That the effect does not depend simply on the contact 

 of cold air, is plain from the following experiment. I exposed to the cold a glass 

 jar, with some distilled water, and placed in it 2 thermometers ; one immersed 

 in the water, the other suspended a little above its surface, in the empty part of 

 the jar. The latter sunk faster than the former; but after a certain time, the 

 thermometer above the surface was at 25^, and that in the water at 25%, yet 

 the water continued unfrozen. I perceive too by M. Wilcke's experiments, 

 that in much more intense cold than we usually experience in this country, 

 vessels of water standing within doors in a laboratory are often cooled so far 

 below the freezing point as to become almost full of ice on being made to shoot, 

 though the surface of the water be in no wise defended from the cold air of the 

 laboratory. Oil spread over the surface of water has been found to prevent it 

 from freezing, when other water similarly exposed has had a crust of ice formed 

 on it. This I ascribe entirely to the prevention of frozen particles from coming 

 in contact with the water : for in experiments with frigorific mixtures, in a room 

 of modercfte temperature, I do not find that oil on the surface has any sensible 

 effect in enabling water to support more cold, unless indeed where the operation 

 is otherwise too much precipitated. Also a crack in the tumbler containing the 

 water prevents it from cooling below the point of congelation, a thin film of ice 

 insinuating itself through the crack into contact with the water. And often, in 

 experiments with frigorific mixtures, the congelation is brought on by raising the 

 immersed thermometer a little out of the water, and lowering it down again ; 

 some of the adhering water having frozen on its stem. 



Several other circumstances, though not so distinctly ascertained as the pre- 

 ceding, appear to facilitate the congelation of cooled water. For instance, in 

 experiments with frigorific mixtures, if the cold be very intense, the water 

 freezes almost immediately round the sides of the vessel, as if something de- 

 pended on too sudden a change of temperature. Accordingly, the only way of 

 insuring the greatest degree of cold in water without freezing, is to cool it in a 

 very gradual manner, keeping the cold of the frigorific mixture regularly only 

 2 or 3° below that of the water. Sudden cooling therefore may be considered 

 as one of the causes which hasten congelation. No doubt this will sometimes 

 depend on such a cold as water cannot resist without freezing, being propagated 

 through the glass to the nearest part of the water, quicker than it can be distri- 



