EVAPORATION 311 



one pound weight of water ; whereas 32 cubic feet of steam of the density corre- 

 sponding to a temperature of 212 and a pressure of 30 inches of mercury, weigh one 

 pound. When the temperature of the water is given, the elasticity and specific gravity 

 of the vapour emitted by it may be found. 



Since the vapour rises from the water only in virtue of the elasticity duo to its 

 gaseous nature, it is obvious that no more can be produced, unless what is already 

 incumbent upon the liquid have its tension abated, or be withdrawn by some means. 

 Suppose the temperature of the water to be midway between freezing and boiling, viz. 

 122 F., as also that of the air in contact with it to be the same, but replete' with 

 moisture, so that its interstitial spaces are filled with vapour of corresponding elasticity 

 and specific gravity with that given off by the water, it is certain that no fresh forma- 

 tion of vapour can take place in these circumstances. But the moment a portion of 

 vapour is allowed to escape, or is drawn off by condensation to another vessel, an 

 equivalent portion of vapour will be immediately exhaled from the water. 



The pressure of the air and of other vapours upon the surface of water in an open 

 vessel, does not prevent evaporation of the liquid ; it merely retards its progress. Ex- 

 perience shows that the space filled with an elastic fluid, as air or other gaseous body, 

 is capable of receiving as much aqueous vapour as if it were vacuous, only the reple- 

 tion of that space with the vapour proceeds more slowly in the former predicament 

 than in the latter, but in. both cases it arrives eventually at the same pitch. Dr. 

 Dalton very ingeniously proved that the particles of aeriform bodies present no per- 

 manent obstacle to the introduction of a gaseous atmosphere of another kind among 

 them, but merely obstruct its diffusion momentarily, as if by a species of friction. 

 Hence, exhalation at atmospheric temperatures is promoted by the mechanical dif- 

 fusion of the vapours through the air with ventilating fans or chimney draughts ; 

 though, under brisk ebullition, the force of the steam readily overcomes that mechanical 

 obstruction. 



The quantities of water evaporated under different temperatures in like times, are 

 proportional to the elasticities of the steam corresponding to these temperatures. A 

 vessel of boiling water exposing a square foot of surface to the fire, evaporates about 

 725 grains in the minute ; the elasticity of the vapour is equivalent to 30 inches of 

 mercury. To find the quantity that would be evaporated from the same surface per 

 minute at a heat of 88 F. : At this temperature the steam incumbent upon water is 

 capable of supporting 1*21 inch of mercury; whence the rule of proportion is 30 : 

 1'28::725 : 30-93; showing that about 31 grains of water would be evaporated in 

 the minute. If the air contains already some aqueous vapour, as it commonly does, 

 then the quantity of evaporation will be proportional to the difference between the 

 elastic force of that vapour, and what rises from the water. 



Suppose the air to be in the hygrometric state denoted by 0'38 of an inch of 

 mercury, then the above formula will become 30 : T28 0'38::725 : 21*41; show- 

 ing that not more than 21 grains would be evaporated per minute under these 

 circumstances. 



The elastic tension of the atmospheric vapour is readily ascertained by the old ex- 

 periment of Le Roi, which consists in filling a glass cylinder (a narrow tumbler for 

 example) with cold spring water, and noting its temperature at the instant it becomes 

 so warm that dew ceases to be deposited upon it. This temperature is that which 

 corresponds to the elastic tension of the atmospheric vapour. 



Whenever the elasticity of the vapour, corresponding to the temperature of the 

 water, is greater than the atmospheric pressure, the evaporation will take place not 

 only from its surface, but from every point in its interior; the liquid particles 

 throughout the mass assuming the gaseous form, as rapidly as they are actuated by 

 the heat which subverts the hydrostatic equilibrium anlong them, to constitute the 

 phenomena of ebullition. This turbulent vaporisation takes place at any temperature, 

 even down to the freezing point, provided the pneumatic pressure be removed from 

 the liquid by the air-pump, or any other means. Ebullition always accelerates eva- 

 poration, as it serves to carry off the aqueous particles not simply from the surface, 

 but from the whole body of the water. 



The vapours exhaled from a liquid at any temperature contain more heat than the 

 fluid from which they spring; and they cease to form whenever the supply of heat 

 into the liquid is stopped. Any volume of water requires for its conversion into 

 vapour about Jive times as much heat as is sufficient to heat it from the freezing to the 

 boiling temperature. The heat, in the former case, seems to be absorbed, being inap- 

 preciable by the thermometer ; for steam is no hotter than the boiling water from 

 which it rises. It has been therefore called by Dr. Black, latent heat-, in contradis- 

 tinction to that perceived by the touch and measured by the thermometer, which is 

 called sensible heat. The quantity of heat absorbed by one volume of water in its con- 

 version into steam, is about 1.000 F. ; it would be inadequate to heat 1,000 volumes 



