HEAT. 



53 



quantities of fluids : in one of these, the 

 end of a pipe conveying steam from the 

 boiler is immersed in the liquid to be 

 heated ; the steam issuing from the pipe 

 is condensed, and, its latent heat being 

 set free, raises the temperature of the 

 liquid. When steam is applied in this 

 way to the contents of vessels, they can- 

 not be heated higher than within two or 

 three degrees of the boiling point, al- 

 though the water seems to boil : the 

 quantity of water must also be greatly 

 increased by the condensation of the 

 steam, in this way of applying it ; and 

 this circumstance would be unfavour- 

 able to some processes. The other way 

 of applying it is to enclose the vessel, 

 steam tight, within another, permitting 

 steam to occupy the space between. In 

 this way the water may be brought to a 

 boiling heat, much sooner than if the 

 vessefcontaining it were placed over an 

 open fire. 



Mr. Parkes made an experiment with 

 a vessel of this kind, intended for pre- 

 paring colours : its diameter, at the top, 

 was twenty inches ; it was eighteen 

 inches deep, in the form of an inverted 

 bee-hive, and calculated to hold twenty 

 gallons. Being filled with cold water, 

 at 52 Fahrenheit, steam was admitted 

 into the space between the two vessels. 

 In six minutes the temperature of the 

 twenty gallons of water was raised to 

 190, in eight minutes to 200, in ten 

 minutes to 208, and in eleven minutes 

 to 212. Parkes Chemical Essays, 

 vol. v. p. 46. 



It is estimated that one gallon of 

 water converted into steam, will heat 

 six gallons at 50 to the boiling point, or 

 eighteen gallons from 50 to 100, mak- 

 ing abundant allowance for waste. 



The heat imparted to the air by steam- 

 pipes is peculiarly elegible for warming 

 apartments ; diffusing a genial and de~ 

 lightful warmth, combining comfort with 

 cleanliness and safety. Calculations 

 applied to the warming* of manufactories 

 by steam, make it appear that one cubic 

 foot of boiler will heat about two thou- 

 sand cubic feet of space, to the tempera- 

 ture of 70 or 80 ; and that one foot of 

 surface of steam-pipe is sufficient to 

 warm 200 cubic feet of space. Cast- 

 iron pipes are considered best for diffus- 

 ing heat, and they should be placed near 

 the floor. 



Steam is a most effectual and con- 

 venient agent for preparing food. A 

 close boiler, of any kind, being con- 

 nected with a range of vessels intended 



for cooking, steam will rush into them, 

 and heat them to the boiling tempera- 

 ture, and maintain them at the same 

 as long as the water in the boiler is 

 kept in a state of ebullition. Animal 

 food prepared in this way is considered 

 by the best judges to be more nutriti- 

 ous and easier of digestion than the 

 same kind of food boiled in the usual 

 way. 



Evaporation. 



The doctrine of latent heat is beauti- 

 fully illustrated by the effects of evapo- 

 ration. Since fluids, when they evapo- 

 rate, require that large quantities of 

 sensible heat should enter into them, 

 and become latent, to enable them to 

 assume the expanded state of vapour, 

 all evaporation produces cold. On this 

 account, showers in summer cool and 

 refresh the earth, because the water 

 which is spread over its surface evapo- 

 rates quickly, and carries aw r ay with it 

 the superfluous heat. 



Ex. The experiment of Dr. Cullen, 

 already alluded to, by which water was 

 frozen round a phial of ether, under a 

 receiver, exhausted by the air-pump, 

 illustrates the effect of evaporation in 

 producing cold. The experiment is now 

 usually performed by setting a watch- 

 glass containing water upon a stand, 

 under the receiver of an air-pump : a 

 thin metallic cup, containing ether, is 

 placed in the middle of the watch glass. 

 As the exhaustion proceeds the rapid 

 evaporation of the ether makes it boil, 

 and the water to freeze at the same 

 time ; so that the metallic cup will, in 

 a short time, be fixed in the centre of a 

 cake of ice. 



Fluids evaporate in vacuo at degrees 

 of temperature from 120 to 12 5 below 

 their boiling point, when exposed to at- 

 mospherical pressure. Advantage has 

 been taken of this circumstance in 

 Howard's plan for refining sugar, and 

 in plans for preparing pharmaceutical 

 extracts in vacuo. The boiler, or eva- 

 porating vessel, of Mr. Barry's appara- 

 tus for this purpose, is connected by a 

 pipe, having a stop-cock, with a copper 

 sphere, three or four times as large as 

 itself. When the juice or infusion to be 

 inspissated has been introduced into the 

 boiler or still, it is closed air-tight, the 

 stop-cock between the boiler and the 

 copper sphere being shut : steam is then 

 introduced into the latter until it issues 

 Out at an aperture uncondensed, when, 

 the aperture being closed, the steam is 



