Let us now inquire what the latent heat of steam is.  If we take 1 kilogram of water at 0 deg.  C. and blow steam from boiling water at 100 deg.  C. into it until the water just boils, and then stop and weigh the resulting water, we shall find it amounts to 1.187 kilograms, so that 0.187 kilogram of water which was in the gaseous steam form, and had besides a sensible heat of 100 deg.  C., has changed its state to that of liquid water.  This liquid water, being at the boiling-point, has still the 100 deg.  C. of sensible heat, and hence the water in the gaseous steam form can have given up to the water at 0 deg.  C. into which it was blown, only the latent heat of gasification which was not sensible, but by virtue of which it was enabled to assume the gaseous form.  But if 0.187 kilogram of steam at 100 deg.  C. can heat 1 kilogram of water through 100 degrees, then 1 kilogram of steam can raise 5.36 kilograms of ice-cold water through 100 degrees, or 536 kilograms through 1 degree, and thus the latent heat of steam is 536 heat units.

Effect of Increase of Pressure on the Boiling of Water.—­Now we have referred to diminution of pressure and its effect on the boiling-point of water, and I may point out that by increasing the pressure, such, e.g., as boiling water under a high pressure of steam, you raise the boiling-point.  There are some industrial operations in which the action of certain boiling solutions is unavailing to effect certain decompositions or other ends when the boiling is carried on under the ordinary atmospheric pressure, and boiling in closed and strong vessels under pressure must be resorted to.  Take as an example the wood-pulp process for making paper from wood shavings.  Boiling in open pans with caustic soda lye is insufficient to reduce the wood to pulp, and so boiling in strong vessels under pressure is adopted.  The temperature of the solution rises far above 212 deg.  F. (100 deg.  C.).  Let us see what may result chemically from the attainment of such high temperatures of water in our steam boilers working under high pressures.  If you blow ordinary steam at 212 deg.  F. or 100 deg.  C., into fats or oils, the fats and oils remain undecomposed; but suppose you let fatty and oily matters of animal or vegetable origin, such as lubricants, get into your boiler feed-water and so into your boiler, what will happen?  I have only to tell you that a process is patented for decomposing fats with superheated steam, to drive or distil over the admixed fatty acids and glycerin, in order to show you that in your boilers such greasy matters will be more or less decomposed.  Fats are neutral as fats, and will not injure the iron of the boilers; but once decompose them and they are split up into an acid called a fat acid, and glycerin.  That fat acid at the high temperature soon attacks your boilers and pipes, and eats away the iron.  That is one of the curious results that may follow at such high temperatures.  Mineral or hydrocarbon oils do not contain these fat acids, and so cannot possibly, even with high-pressure steam, corrode the boiler metal.