VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 585 



ing part of the aqueous fluid off with it. We are hence furnished with a fresh 

 and continual source of vapour as well as of heat ; so that the processes of lique- 

 faction and congelation may go on uninterruptedly together, and even necessarily 

 accompany each other, though, as the freezing must be in an under proportion 

 to the melting, the whole of the ice must ultimately be consumed. 



In the remarkable instance of the coating of ice on the outside of the throat 

 of the funnel, there are some other circumstances which it may be proper to 

 take notice of. Neither the cover of the outer vessel, nor the aperture in its 

 bottom which the stem of the funnel passed through, were air-tight, and the 

 melting of the surrounding ice had left a vacancy of about an inch round that 

 part of the funnel on which the crust had formed. As there was therefore a pas- 

 sage for air through the vessel, a circulation of it would probably take place : 

 the cold and dense air in the vessel would descend into the rarer air of the room 

 then about 50°, and be replaced by air from above. The effect of this circula- 

 tion and sudden refrigeration of the air will be a condensation of part of the 

 moisture it contains on the bodies it is in contact with : the throat of the funnel, 

 being one of those bodies, must receive its share ; and the degree of cold in 

 which the ice thaws being supposed sufficient for the freezing of this moist 

 vapour, the contact, condensation, and freezing, may happen at the same in- 

 stant. The same principles apply to every instance of congelation that took 

 place in these experiments ; and a recollection of particulars which passed under 

 my own eye convinces me, that the congelation was strongest in those circum- 

 stances where vapour was most abundant, and on those bodies which, from their 

 natural or mechanic structure, were capacious of the greatest quantity of it ; 

 stronger, for instance, on sponge than on woollen, stronger on this than on the 

 closer texture of linen, and far stronger on all these than on the compact sur- 

 face of porcelain. 



However, if the principle I have assumed (that water highly attenuated will 

 congeal with a less degree of cold than water in the mass) should not be admit- 

 ted ; another has above been hinted at, which experiments have decidedly esta- 

 blished, from which the phenomena may perhaps be equally accounted for, and 

 which, even though the other also is received, must be supposed to concur for 

 some part of the effect ; I mean, that evaporation produces cold ; both vapour 

 and steam carrying off some proportion of heat from the body which produces 

 them. If therefore evaporation be made to take place on the surface of ice, the 

 contiguous ice will thus be rendered colder ; and as it is already at the freezing 

 point, the smallest increase of cold will be sufficient for fresh congelation. It 

 seems to be on this principle that the formation of ice is effected in the East 

 Indies, by exposing water to a serene air, at the coldest season of the year, in 

 shallow porous earthen vessels : part of the water transudes through the vessel, 



vol. xv. 4 F 



