HEAT. 



Fig. 16 



pared with that which would result 

 from the addition of an equal weight of 

 boiling water ; and it will be found that 

 a given weight of steam, at 212 3 , has 

 the power of heating water many times 

 more than an equal weight of water at 

 the same temperature. The thermo- 

 meter c passes through a collar into the 

 boiler a, for the purpose of ascertaining 

 the heat of its contents. 



Dr. Ure remarks that it is the greatly 

 superior relation to heat which steam 

 possesses above water, that makes the 

 boiling point of that fluid so perfectly 

 stationary, in open vessels, over the 

 strongest fires ; and he found that va- 

 pours which have less latent heat are 

 not capable, by their formation, of 

 keeping their respective liquids while 

 boiling at the same uniform tempera- 

 tures. This he found to be the case 

 with oil of turpentine, petroleum and 

 sulphuric acid, which being heated 

 briskly in common glass phials, they 

 rose from twenty to thirty degrees above 

 the points at which they boiled in 

 hemispherical capsules. 



The gases being similar to vapour in 

 their high relation to heat, when they 

 enter into liquid or solid states, heat 

 is copiously evolved. The fixation of 

 gaseous matter in the burning of bodies, 

 occasioning the evolution of heat, was 

 believed by Lavoisier and others to be 

 the sole source of heat in the process of 

 combustion. The cause of ebullition, or 

 boiling, is the formation of vapour at 



the bottom of a vessel, in consequence of 

 the application of heat there ; the vapour 

 being so much lighter than the fluid, 

 bubbles of it continually rise to the sur- 

 face and escape ; the passage of these 

 vapour bubbles through the water pro- 

 duces that agitation which is called 

 boiling, or ebullition. Under the usual 

 pressure of the air, water boils near the 

 level of the sea at 212 of Fahrenheit; 

 but when that pressure is reduced, less 

 heat is sufficient to produce ebullition ; 

 as when water is carried up the side of 

 a mountain, the greater the height, the 

 less heat will be required to make the 

 water boil, because a great proportion 

 of the heavy column of air which occa,- 

 sions the pressure is left beneath. Upon 

 this principle the thermometric baro- 

 meter of the Rev. Mr. Wollaston is 

 constructed, which indicates the eleva- 

 tion of any place above the level of the 

 sea, by the difference in the heat re- 

 quired to make water boil at that ele- 

 vation. A difference of one degree 

 in the boiling point of water is occa- 

 sioned by a difference in height, which 

 lowers the barometer 0.589 of an 

 inch, and corresponds very nearly to a 

 difference of elevation amounting to 

 520 feet. 



Saussure found that water boils at 

 the top of Mont Blanc when heated to 

 187 degrees. 



The boiling point of water differs ac- 

 cording to the state of the ah-: when 

 the barometer stands at 31 inches, more 



E 



