58 1 PHILOSOPHICAL TRANSACTIONS. [ANNO 1/84. 



fullest evidence that my senses can give me ; and I shall take the liberty of sug- 

 gesting a few hints, which may tend perhaps to elucidate their cause, and to 

 show that they are not so incompatible as at first sight they appear to be. It 

 occurred to me at first, that water highly attenuated and divided, as when re- 

 duced into vapour, may freeze with a less degree of cold than water in its aggre- 

 gate or grosser form ; hence hoar-frost is observed on grass, trees, &c. at times 

 when there is no appearance of ice on water, and when the thermometer is above 

 the freezing point.* Boerhaave, I find, in his elaborate theory of fire, assigns 

 33° as the freezing point of vapour, and even of water when divided only by 

 being imbibed in, a linen cloth. 



Now, as the atmosphere abounds with watery vapour, or water dissolved and 

 chemically combined, and must be particularly loaded with it in the neighbour- 

 hood of melting ice ; as the heated body introduced into the funnel must neces- 

 sarily convert a portion of the ice or water there into vapour ; and as ice is 

 known to melt as soon as the heat begins to exceed 32°, or nearly 1° lower than 

 the freezing point of vapour ; I think we may hence deduce, pretty satisfactorily, 

 all the phenomena I have observed. For it naturally follows from these princi- 

 ples, that vapour may freeze where ice is melting ; that the vapour may congeal 

 even on the surface of the melting ice itself; and that the heat which, agreeably 

 to the ingenious theory of Dr. Black, it emits in freezing, may contribute to 

 the further liquefaction of that very ice on which the new congelation is formed. 



I would further observe, that the freezing of water is attended with a plentiful 

 evaporation in a close as well as an open vessel, the vapour in the former con- 

 densing into drops on the under side of the cover, which either continue in the 

 form of water, or assume that of ice or a kind of snow, according to circum- 

 stances ;-f which evaporation may perhaps be attributed to the heat that was 

 combined with the water, at this moment rapidly making its escape, and carry- 



* I am aware that experiments and observations of this kind are not fully decisive ; that the at- 

 mosphere may, in certain circumstances, be much warmer or colder than the earth and waters, 

 which, in virtue of their density, are far more retentive of the temperature they have once received, 

 and less susceptible of transient impressions ; that even insensible undulations of water, from the 

 slightest motion of the air, by bringing up warmer surfaces from below, may prove a further im- 

 pediment to the freezing ; and therefore, that the degree of cold, which is sufficient to produce 

 hoar-frost, may possibly, if continued long enough, be sufficient also to produce ice. I am not ac- 

 quainted with any satisfactory experiments or observations yet made on the subject ; nor do I advance 

 the principle as a certain, but as a probable one, which occurred to me at the moment, u Inch i-; 

 countenanced by general observation, and consentaneous to many known facts; for there are nume- 

 rous instances of bodies, in an extreme state of division, yielding easily to chemical agents which, 

 before such division, they entirely resist : thus some precipitates, in the very subtile state in which 

 they are at first extricated from their dissolvents, are re-dissolved by other menstrua, which, after 

 their concretion into sensible moleculse, have no action on them at all. — Orig. 



\ See Mr. Baron's paper on this subject, in the Paris Memoires for the year 1773. — Orig. 



