166. Q.—­Have not some superior experiments upon this subject been lately made in France?

A.—­Yes, the experiments of M. Regnault upon this subject have been very elaborate and very carefully conducted, and the results are probably more accurate than have been heretofore obtained.  Nevertheless, it is questionable how far it is advisable to disturb the rules of Watt and Southern, with which the practice of engineers is very much identified, for the sake of emendations which are not of such magnitude as to influence materially the practical result.  M. Regnault has shown that the total amount of heat, existing in a given weight of steam, increases slightly with the pressure, so that the sum of the latent and sensible heats do not form a constant quantity.  Thus, in steam of the atmospheric pressure, or with 14.7 Lbs. upon the square inch, the sensible heat of the steam is 212 degrees, the latent heat 966.6 degrees, and the sum of the latent and sensible heats 1178.6 degrees; whereas in steam of 90 pounds upon the square inch the sensible heat is 320.2 degrees, the latent heat 891.4 degrees, and the sum of the latent and sensible heats 1211.0 degrees.  There is, therefore, 33 degrees less of heat in any given weight of water, raised into steam of the atmospheric pressure, than if raised into steam of 90 Lbs.[1] pressure.

167. Q.—­What expansion does water undergo in its conversion into steam?

A.—­A cubic inch of water makes about a cubic foot of steam of the atmospheric pressure.

168. Q.—­And how much at a higher pressure?

A.—­That depends upon what the pressure is.  But the proportion is easily ascertained, for the pressure and the bulk of a given quantity of steam, as of air or any other elastic fluid, are always inversely proportional to one another.  Thus if a cubic inch of water makes a cubic foot of steam, with the pressure of one atmosphere, it will make half a cubic foot with the pressure of two atmospheres, a third of a cubic foot with the pressure of three atmospheres, and so on in all other proportions.  High pressure steam indeed is just low pressure steam forced into a less space, and the pressure will always be great in the proportion in which the space is contracted.

169. Q.—­If this be so, the quantity of heat in a given weight of steam must be nearly 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.