Y-i •*•'' ExphMotiWef-fhe'Pnef/icfFi-ffiiiig in IfnUr. 



extreme, ami there appears to be no fourcc of heat left which is fufficiciit to moderate it 

 in any fenfiblc dcgrrc. 



It has been Ihewn — the Count thinks he may venture to Giy proved — in the mod fatisfac- 

 fory manner, that liquids part «ith their heat only in confequence of their internal motions, 

 and that t!ie more rapid thefe motions are, the more rapid is tlic communication of the heat ; 

 that thcfc motions are produced by the change in the fpecific gravity of the liquid occa- 

 foncd by the change of temperature ; and of courfe that they arc more rapid as the fpecific 

 gravity of the liquid is more changed by any given change of temperature. 



But it has been fhown that the change in the fptcific gravity of water is extremely fm^ll, 

 which takes place in any given change of temperature below the mean temperature of the 

 atmofphere ; and particularly when tlie temperature of the water is very near the freezing 

 point; and hence it follows that water mufl give olF its heat very flowly wlien it is 

 near freezing. 



Cut this is not all. When water is cooled to within eight or nine degrees of the freez- 

 ing point, it not only ceafes to be farther condenfcd, but is aflnally expanded by farther di- 

 minutions of its heat ; and this expanfion goes on, as the heat is diminilhed, as long as the 

 water can be kept fluid ; and when it is changed to ice it expamis even dill more, and the 

 fee floats on the furface of the uncongealcd part of the fluid. 



It is well known that there is no communication of licat between two bodies, as long as 

 they arc both at the fame temperature ; and it is likewife known that the tendency of 

 heat to pafs from a hot body into one which is colder, with which it is in cont.itl, is 

 greater, as the difference is greater in the temperatures of the two bodies. 



Suppjfe now that a mafs of very cold air repofes on the quiet furface of a large lake of 

 frcfli water, at the temperature of 5 50 of Fabrenhell's thermometer. The particles of wa- 

 ter at the furface, on giving off a part of their heat to the cold air with which they are in 

 contafl, and in confequence of this lofs of heat becoming fpecifically heavier than thofe hotter 

 particles on wliich they repofe, mufl of courfe defcend. This defcent of the particles which 

 have been cooled, ncceffarlly forces other hotter particles to the fuiface ; and thefe being 

 cooled in their turns, bend their courfe downwards ; and the whole mafs of water is put 

 ■into motion, and continues in motion as long as the procefs of cooling goes on. 



Before he proceeds to trace this operation through all its various ftages, the Count en- 

 deavours to remove an objection which may perhaps be made to his explanation of this 

 phenomenon. As he fuppofes the mafs of air which reds on the furface of the water to 

 be very cold, and as he has taken it for granted that there is no communication whatever of 

 heat between the particles of water in contafl with this very cold air, and the neighbouring 

 warmer particles of water, it may be afked, how it happens that thefe particles at the furface 

 are not fo much cooled as to be immediately changed to ice .' To this he anfwcrs, that there 

 are two caufes which confpire to prevent the immcJiate formation of ice at the furface of 

 the water. Pirft, the fpecific gravity of the particle of water at the furface being increafed 

 at the fame moment when it parts with heat, it begins to defcend as foon as it begins to be 

 cooled j and before the air has had time to rob it of all its heat, it efcapes and gets out of 

 its reach : and. Secondly, air being a bad conducftor of heat, it cannot receive and tranfmit, 

 or tranfport it with fufficicnt celerity to cool the furface of water fo fuddenly as' to embarrafs 

 the motions of the particles of that liquid in the operation of giving it off. ' 



