ON EYAJPORATION^ 15 



very little. Now ether in the above experiments is a6led upon 

 by a current of air of an equal or higher temperature than it- 

 felf; but water of 1 60*» is ufually aded upon by air 100^ lower 

 than itfelf, which is every moment precipitating the vapour 

 formed, and thus obftruding its circulation. This appears to 

 be a fufficient caufe for the fmall difference obferved. 



With refpefl to mercury, fulphuric acid, muriate of lime. Evaporation of 

 &c. there can be no doubt but they experience a real evapo- '""^^'^'^y* *<^' 

 ifation like thofe above; but it muft be very fmall in proportion 

 as their boiling points are high. And it would be difficult to 

 make experiments upon fuch of thefe as have an affinity for 

 aqueous vapour; becaufe their acquiiitlon from the aqueous at- 

 mofphere would far exceed their lofs by evaporation. 



Since writing the above cflay, opportunities have occurred J^^ evaporation 

 to afcertain whether the evaporation from ice is conformable to with the pr«cc- 

 the fame law as that from water. Every one, who has tried ^'"S ^^* 

 the experiment, admits the faft that ice is evaporable. I have 

 lately made feveral obfervations on this fubje<5l, the refults of 

 which, as far as they go, fupport the conclufion that the ge- 

 neral law of evaporation continues the fame below the point 

 of congelation as above it. All the experiments were made 

 in the tin veflel above defcribed of fix inches in diameter; a 

 quantity of water was fuffered to freeze in it, fo as to form a 

 circular cake of ice; the veflel and ice were then weighed to- 

 gether, and expofed in the open air for a certain time, after, 

 which being again weighed, the lofs was found ; the force of 

 the aqueous atmofphere was foraetimes determined during the 

 experiment by a mixture of pounded ice and fait, in the man- 

 ner already defcribed. 



Grs. H. Grs. Wind. Air. 



J»Iov. 5. In the night loft 110 In 9 j or, .20 per m. N. E. briik. »8<? to 3i<> Experimental 



— at 10 A. M. — . 25 in il; or, .33 N. E. mod. 31° refults. 



—•29. at 1 P. M. — 24 in i|i or, .23 calm. 31** 



P. M. — 84 in 9I; or, ,15 — — 30* 



—30. In the night — 94 in 9 j or, .77 — — N. E, mod. 31' 



Dec. 19. P. M. — 75 in 8 $ or, .16 N. E. calm. 26** —28' 



In the night — 33 in ii j or, »05 — — < calm. 29* ' 



•—20. A. M. — 21 in 2 J or, .175 — — W. mod 31* 



Some of thefe being made in the night, and of long duras 

 tion, neither the temperature of the air, nor the force of the 

 aqueous atmofpheie could be fairly determined : the fecond 



experiment 



