VOL. LXXIII.J PHILOSOPHICAL TRANSACTIONS. 423 



Boiling point . Freezing point. 



a 220.3 29-9 



b 218.8 30.9 



G 215.3 32. 



From what has been said it appears, that 183°.3 on the scale of g, are equal 

 to only 180° on a thermometer adjusted as recommended by the committee, and 

 therefore 72° are equal to 70°-*- ; so that the point of — 40° answers really to 



— 38°a ; that is, the cold shown by this thermometer at the temperature of about 



— 40°, is 1% too great. In like manner it appears that the cold shown at that 

 temperature by e is 4%, and by a 6±, too great. 



Before entering on the examination of Mr. Hutchins's experiments, it will be 

 proper to take notice of a phenomenon which occurs in the freezing of water, 

 and is now found to take place in that of quicksilver, and which occasioned many 

 remarkable appearances in these experiments. It is well known, that if a 

 vessel of water, with a thermometer in it, be exposed to the cold, the ther- 

 mometer will sink several degrees below the freezing point, especially if the 

 water is covered up so as to be defended from the wind, and care is taken not to 

 agitate it ; and then, on dropping in a bit of ice, or on mere agitation, spiculae 

 of ice shoot suddenly through the water, and the inclosed thermometer rises 

 quickly to the freezing point where it remains stationary. 



This shows that water is capable of being cooled considerably below the freez- 

 ing point, without any congelation taking place ; and that as soon as by any 

 means a small part of it is made to freeze, the ice spreads rapidly through the 

 remainder of the water. The cause of the rise of the thermometer, when the 

 water begins to freeze, is the circumstance now pretty well known to philoso- 

 phers, that all, or almost all bodies, by changing from a fluid to a solid state, or 

 from the state of an elastic to that of an unelastic fluid, generate heat ; and 

 that cold is produced by the contrary process. This explains all the circum- 

 stances of the phenomenon perfectly well ; for as soon as any part of the water 

 freezes, heat will be generated in consequence of the abovementioned law, so 

 that the new formed ice and remaining water will be warmed, and must continue 

 to receive heat by the freezing of fresh portions of water, till it is heated exactly 

 to the freezing point, unless the water would become quite solid before a suffi- 

 cient quantity of heat was generated to raise it to that point, which is not the 

 case ; and it is evident that it cannot be heated above the freezing point, for as 

 soon as it comes to it, no more water will freeze, and consequently no more heat 

 will be generated. The reason why the ice spreads all over the water, instead of 

 forming a solid lump in one part, is, that as soon as any small portion of ice is 

 formed, the water in contact with it will be so much warmed as to be prevented 

 freezing ; but the water at a little distance from it will still be below the freezing 

 point, and will consequently begin to freeze. 



