90 HEAT OR CALORIC. 



Pressure in inches Temperature* 



Atmospheres. of mercury. P'ahr. 



1 29.8 - 212.0 



2 - - 59.6 - - 250.3 

 4 - 179.2 - 293.4 

 8 - 238.4 - - 343.6 



(q.) The latent heat of steam may be shewn by the digester. Five- 

 gallons of water are heated to 400 ; the orifice being opened, one 

 gallon flies away in the form of steam ; the resulting temperature is 

 212 ; therefore one gallon in steam has carried away heat repre- 

 sented by 5 X 188 940= nearly the latent heat of steam; for 400 C 

 212 -- 188, and there were five gallons of water. 



(r.) The latent heat of condensed steam, if suffered to pads into 

 cold water, makes it boil quickly, and it soon melts ice. Great noise 

 is produced by steam striking' cold water; this is owing to its sud- 

 den condensation, and the noise grows less as the water becomes 

 hotter, till finally the steam passes almost silently through water, at or 

 near 212, like a gas, and is not condensed.* 



The better way to heat water, is to surround by steam, the vessel 

 containing the water to be heated. Mr. Parkes heated twenty gal- 

 lons in this manner, in six minutes, from 52 to 190, in eight minutes 

 to 200, in ten minutes to 208, and in eleven to 212. L. u. K.f 



High steam does not scald, because it is cooled by its sudden ex- 

 pansion, and it blows along with it a mass of cold air ; indeed it is no 

 longer high steam, but common steam partly condensed. It also blows 

 a burning brand powerfully, but if held too near, it extinguishes the 

 fire in consequence of the condensation of the steam ; it does not 

 scald the hand, at a few inches from the orifice. The agent in the 

 combustion is not so much the steam as the air which it blows along ; 

 still, at a very high temperature, the steam may be, and probably is 

 decomposed, giving oxygen to the carbon, and hydrogen to the flame. 



There is a popular impression that a boiling tea kettle does not 

 burn the hand, but that, if it ceases boiling, it will produce that effect ; 

 perhaps there is a mistake in the fact ; and this is the more proba- 

 ble, as the trial is of course made in a hurried and imperfect manner. 



(s.) The density of steam confined over water, is directly as its 

 elasticity ; that is, the higher the temperature and the greater the 

 elasticity, the greater is the quantity of water contained in steam of 

 the same volume. J 



* It is said however that water heated in this way is still two or three degrees 

 short of the boiling point. L. tr. K. 

 t Quoting Parkes' Chern. Essay. 

 t Henry, Vol. I, p. 122, Lond. Ed. 10, 



