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VOL. LXV.] PHILOSOPHICAL TRANSACTIONS. . 6Qg 



distant from the cockle than from the wall of the room. In about 20 minutes 

 the eggs were taken out, roasted quite hard ; and in 47 minutes the steak was 

 not only dressed, but almost dry. Another beef-steak was rather over-done in 

 33 minutes. In the evening, when the heat was still greater, we laid a 3d beef- 

 steak in tlie same place : and as it had now been observed, that the effect of the 

 heated air was much increased by putting it in motion, we blew upon the steak 

 with a pair of bellows, which produced a visible change on its surface, and 

 seemed to hasten the dressing ; the greatest part of it was found pretty well done 

 in thirteen minutes. 



About the middle of the day 2 similar earthen vessels, 1 containing pure 

 water, and the other an equal quantity of the same water with a bit of wax, were 

 put upon a piece of wood in the heated room. In li hour the pure water was 

 heated to 140° of the thermometer, while that with the wax had acquired a heat 

 of 152°, part of the wax having melted and formed a film on the surface of the 

 water, which prevented the evaporation. The pure water never came near the 

 boiling point, but continued stationary above an hour at a much lower degree ; 

 a small quantity of oil was then dropped into it, as had before been done to that 

 with the wax ; in consequence of which, the water in both the vessels came at 

 length to boil very briskly. A saturated solution of salt in water, put into the 

 room, was found to heat more quickly, and to a higher degree, than pure water, 

 probably because it evaporated less ; but it could not be brought to boil till oil 

 was added, by means of which it came towards evening into brisk ebullition, 

 and consequently had acquired a heat of 230°. Some rectified spirit of wine in 

 a bottle slightly corked, which had been immersed into this solution of salt while 

 cold, began to boil in about 2 hours, and soon afterwards was totally evaporated. 

 Perhaps no experiments hitherto made, furnish more remarkable instances of the 

 cooling effect of evaporation, than these last facts ; a power which appears to be 

 much greater than has commonly been suspected. The evaporation itself, how- 

 ever, was more considerable in our experiments than it can be in almost any other 

 situation, because the air applied to the evaporating surface was uncommonly 

 hot, and at the sanie time not more charged with moisture than in its ordinary 

 state. A powerful assistant evaporation must undoubtedly prove, in keeping the 

 living body properly cool, when exposed to great heats ; but it can act only in a 

 gross way, and by no means in such a nice proportion to the momentary exigen- 

 cies of the animal, as would be requisite for the exact preservation of its teni- 

 perature : that other provision of nature which seems more immediately con- 

 nected with the powers of life, is probably the great agent in preserving the just 

 balance of temperature ; exerting a greater effort in proportion as the evaporation 

 is deficient, and a less effort as the evaporation increases. This idea corresponds 

 with the general analogy of the animal economy, the nicer balances of which 



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