HEAT. 



that all the receivers might be exhausted 

 at the same time. Another and more 

 effectual plan was to fill a cast-iron 

 cylinder or drum, of considerable di- 

 mensions, with steam, by boiling a 

 small quantity of water in it ; the air 

 being expelled, the steam is condensed 

 by cold water. A vessel of sulphuric 

 acid, and another of water, being; placed 

 under a receiver, upon a plate attached 

 to the drum by a pipe with a stop- 

 cock, and a communication made with 

 the exhausted drum, the air in the re- 

 ceiver will be rarefied so as to occasion 

 the congelation of the water. If th e 

 vacuum in the cylinder could be made 

 nearly perfect, its size being sixty times 

 greater than that of the receiver,' the air 

 within the latter would be rarefied sixty 

 times ; and the moisture of the cylinder 

 would be excluded by turning the stop- 

 cock again, after the communication 

 had been effected. 



The effect of evaporation in with- 

 drawing heat is admirably illustrated 

 by the process of perspiration. The 

 natural temperature of the human body 

 is from 96 to 98 ; but when we take 

 very active exercise, or when we are 

 exposed to a great degree of heat, 

 there is a tendency to a rise of tempe- 

 rature above that which is conducive 

 to health ; and the most injurious effects 

 would ensue, if they were not prevented 

 by perspiration. Whenever this ten- 

 dency bejjins to be experienced, a 

 watery fluid is brought to the surface 

 of the skin, that, by its evaporation, the 

 body may be cooled to the . healthy 

 emperature. 



Examples of the power of the human 

 body to support heat, under apparently 

 dangerous circumstances, have been 

 placed upon record. Sir Joseph Banks 

 and Sir Charles Blagden, being anxious 

 to ascertain the highest degree of heat 

 that the human body can endure with- 

 out injury, went into an apartment 

 prepared for their reception by Dr. 

 Fordyce. The account given of this 

 experiment by Sir Charles Blagden may 

 be seen in the Philosophical Transac^ 

 tions for 1775, pp. Ill and 484. The 

 temperature of the room was gradually 

 raised until it became 52 hotter than 

 boiling water, as indicated by thermo- 

 meters in different parts of the room. 

 Many persons may feel disposed to 



h doubt the correctness of these indica- 

 tions ; but they were abundantly con- 

 firmed by other circumstances ob- 

 served at the time. The gentlemen 



found that their watch-chains, and all 

 other metallic articles about their per- 

 sons, were so hot that they could not 

 bear to touch them ; eggs placed upon 

 a tin frame were roasted hard in twenty 

 minutes, and a beef-steak was overdone 

 in thirty-three minutes. 



Notwithstanding the extraordinary 

 degree of heat to which the experi- 

 menters were exposed, the temperature 

 of their bodies was not raised. 



It is affirmed that evaporation did 

 not take place from the skin ; but it is 

 difficult to imagine that they could 

 have remained so long under the cir- 

 cumstances described without the pro- 

 tecting influence of perspiration, espe- 

 cially as it is well known that copious 

 perspiration is usually the result of 

 exposure to high degrees of heat. 



Apartments in India are often sepa- 

 rated from the courts by curtains in- 

 stead of walls ; and these curtains, in 

 order to cool the air in the rooms, are 

 continually sprinkled with water, the 

 rapid evaporation resulting occasions 

 a reduction of temperature from 1 to 

 15 degrees. 



The porous vessels of earthenware 

 which are used for wine-coolers act 

 upon the same principle ; being dipped 

 in water, they imbibe a quantity of it, 

 which gradually evaporates ; and, as a 

 part of the heat necessary to convert 

 the water into vapour will be taken 

 from a bottle of wine placed in it, the 

 wine is considerably cooled. 



The people who cross the deserts of 

 Arabia in large parties called Caravans, 

 load camels with earthenware bottles 

 filled with water, and preserve the 

 water at a cool temperature on the 

 principle of evaporation, by wrapping 

 the jars in linen cloths, which they keep 

 moist with water. 



According to Athenaeus, the Egyp- 

 tians availed themselves of the same 

 principle. Servants were employed 

 durins: the night to keep pitchers of 

 water^ constantly wet, that the water 

 might be cooled 'by evaporation ; and to 

 preserve it at the low temperature to 

 which it was thus brought, the pitchers 

 were bound round with straw in the 

 morning. 



Dr. Wollaston has invented an ele- 

 gant instrument for the purpose of il- 

 lustrating the effect of evaporation in 

 producing cold, and calls it the Cryo- 

 phorus, or frost-bearer. It consists of a 

 glass tube, from eighteen inches to two 

 feet in length, having an internal dia- 



