the same, whether the steam is high or low pressure?

_A._--Yes; the heat in steam is nearly a constant quantity, at all
pressures, but not so precisely. Steam to which an additional quantity of
heat has been imparted after leaving the boiler, or as it is called
"surcharged steam," comes under a different law, for the elasticity of such
steam may be increased without any addition being made to its weight; but
surcharged steam is not at present employed for working engines, and it may
therefore be considered in practice that a pound of steam contains very
nearly the same quantity of heat at all pressures.

170. _Q._--Does not the quantity of heat in any body vary with the
temperature?

_A._--Other circumstances remaining the same the quantity of heat in a body
increases with the temperatures.

171. _Q._--And is not high pressure steam hotter than low pressure steam?

_A._--Yes, the temperature of steam rises with the pressure.

172. _Q._--How then comes it, that there is the same quantity of heat in
the same weight of high and low pressure steam, when the high pressure
steam has the highest temperature?

_A._--Because although the temperature or sensible heat rises with the
pressure, the latent heat becomes less in about the same proportion. And as
has been already explained, the latent and sensible heats taken together
make up nearly the same amount at all temperatures; but the amount is
somewhat greater at the higher temperatures. As a damp sponge becomes wet
when subjected to pressure, so warm vapor becomes hot when forced into less
bulk, but in neither case does the quantity of moisture or the quantity of
heat sustain any alteration. Common air becomes so hot by compression that
tinder may be inflamed by it, as is seen in the instrument for producing
instantaneous light by suddenly forcing air into a syringe.

173. _Q._--What law is followed by surcharged steam on the application of
heat?

_A._--The same as that followed by air, in which the increments in volume
are very nearly in the same proportion as the increments in temperature;
and the increment in volume for each degree of increased temperature is
1/490th part of the volume at 32 deg.. A volume of air which, at the
temperature of 32 deg., occupies 100 cubic feet, will at 212 deg. fill a space of
136.73 cubic feet. The volume which air or steam--out of contact with
water--of a given temperature acquires by being heated to a higher
temperature, the pres