HEAT. 



If the flask is taken out before the 

 b oiling ceases, and is plunged into hot 

 water, it will immediately stop boiling, 

 being re-plunged into the cold water the 

 boiling will begin again. 



In this experiment the air is exclu- 

 ded, and the vessel is filled with steam 

 before the cork is introduced. The 

 vessel being removed from the lamp, the 

 water boils a little, because the air of 

 the atmosphere coming into contact 

 with its sides, condenses a portion of 

 the steam within, and thus lessens the 

 pressure upon its surface ; so that it is 

 e nabled to boil, although its tempera- 

 ture be reduced. 



When the flask is plunged into cold 

 water, the steam within, losing its latent 

 heat, is condensed ; and the water, being 

 by its condensation freed from pressure, 

 boils with violence. 



If the flask be now plunged into 

 water, near the boiling point, it will in- 

 stantly cease to boil, because the heat 

 will convert a small portion of the 

 water within the flask into vapour, 

 which, occasioning a new pressure, pre- 

 vents the water from boiling in its cooled 

 state ; or being re-plunged into the cold 

 water, this minute quantity of vapour 

 is condensed, and the water in the flask 

 boils again. 



When fluids are subjected to a greater 

 degree of pressure than that of the at- 

 mosphere, they require higher degrees 

 of heat to make them boil. The ap- 

 paratus, (fig. 18,) is well adapted to 

 illustrate this fact. The hollow globe a 

 is composed of two strong hemispheres 

 of brass, screwed together with flanges : 

 some quicksilver is first poured into 

 this globe, which is then about half 

 filled with water; the barometer tube 

 b passes through a steam-tight collar, 

 and has its end immersed in the quick- 

 silver : c is a thermometer for the pur- 

 pose of ascertaining the temperature of 

 the water during the experiments ; its 

 scale is graduated as high as 400, and 

 passes through a steam-tight collar ; d, 

 is the brass frame and stand upon 

 which the globe rests ; e is a spirit lamp, 

 andf is an aperture closed by a stop- 

 cock. The stop-cock being closed, and 

 heat applied to this vessel, the tempe- 

 rature of the water soon rises above the 

 boiling point. When the heat arrives at 

 '218 degrees the mercury will" be elevated 

 to six inches in the tube, by the elastic 

 pressure of the steam ; at 224 the mer- 

 cury will have ascended twelve inches, 

 rising an inch for every increased de- 



gree of heat. At 242 degrees' the elas- 

 ticity of the steam will balance a 



Fig. 18. 



column of quicksilver thirty inches high, 

 which is equal to the pressure of one 

 atmosphere. 



The facility with which steam imparts 

 its vast treasures of heat to any body 

 colder than itself, fits it admirably for 

 many purposes in domestic life and the 

 arts. Heat applied in this way is much 

 preferable to the heat of a fire, for mak- 

 ing extracts of vegetables in pharmaceu- 

 tical preparations, for heating dyers' and 

 brewers' vats, for the preparation of 

 colours, for warming baths, for drying 

 manufactured goods, for heating apart- 

 ments, and for culinary operations. 



Steam may be applied in two different 

 ways for the purpose of heating large 



