Steam:
Its Generation and Use

THE THEORY OF STEAM MAKING. [Extracts from a Lecture delivered by Geo. H. Babcock, at Cornell University, 1887.*] The chemical compound known as HO exists in three states or conditions — ice, water, and steam; the only difference between these states or conditions is in the presence or absence of a quantity of energy exhibited partly in the form of heat and partly in molecular activity, which, for want of a better name, we are accustomed to call “ latent heatand to transform it from one state to another we have only to supply or extract heat. For instance, if we take a quantity of ice, say one pound, at absolute zerof and supply heat, the first effect is to raise its temperature until it arrives at a point 492 Fahrenheit degrees above the starting point. Here it stops growing warmer, though we keep on adding heat. It, however, changes from ice to water, and when we have added sufficient heat to have made it, had it remained ice, 283° hotter, or a tempera- ture of 3150 by Fahrenheit’s thermometer, it has all become water, at the same temperature at which it commenced to change, namely, 4920 above absolute zero, or 32° by Fahrenheit’s scale. Let us still continue to add heat, and it will now grow warmer again, though at a slower rate — that is, it now takes about double the quantity of heat to raise the pound one degree that it did before — until it reaches a temperature of 2120 Fahrenheit, or 672° absolute (assuming that we are at the level of the sea). Here we find another critical point. However much more heat we may apply, the water, as water, at that pressure, cannot be heated any hotter, but changes on the addition of heat to steam ; and it is not until we have added heat enough to have raised the temperature of the water 966°, or to 1,178 by Fahrenheit’s thermometer (presuming for the moment that its specific heat has not changed since it became water), that it has all become steam, which steam, nevertheless, is at the temperature of 2120, at which the water began to change. Thus over four-fifths of the heat which has been added to the water has disap- peared or become insensible in the steam to any of our instruments. It follows that if we could reduce steam at at- mospheric pressure to water, without loss of heat, the heat stored within it would cause the water to be red hot; and if we could further change it to a solid, like ice, without loss of heat, the solid would be white hot, or hotter than melted steel — it being assumed, of course, that ♦See Scientific American Supplement, 624, 625, Dec. 1887. +460° below the zero of Fahrenheit. This is the nearest approximation in whole degrees to the latest determinations of the absolute zero of temperature. the specific heat of the water and ice remain nor- mal, or the same as they respectively are at the freezing point. After steam has been formed, a further addi- tion of heat increases the temperature again at a much faster ratio to the quantity of head added, which ratio also varies according as we maintain a constant pressure or a constant volume; and I am not aware that any other critical point ex- ists where this will cease to be the fact until we arrive at that very high temperature, known as the point of dissociation, at which it becomes re- solved into its original gases. 1 he heat which has been absorbed by one pound of water to convert it into a pound of steam at atmospheric pressure is sufficient to have melted three pounds of steel or thirteen pounds of gold. This has been transformed into something besides heat; stored up to reap- pear as heat when’the process is reversed. That condition is what w,e are pleased to call latent heat, and in it resides mainly the ability of the heat to temperature, the horizontal scale being quantity of heat in British thermal units, and the vertical temperature in Fahrenheit degrees, both reckoned from absolute zero and by the usual scale. The dotted lines for ice and water show the temperature which would have been obtained if the conditions had not changed. The lines