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of steam, a quantity of latent heat capable of rais-ing a kilogram of water (if prevented from evaporat-ing), from 0 up to 535 centigrade degrees. Thisresult will, no doubt, appear enormous, but it iscertain ; water, as steam, exists only in this con-dition ; wherever a kilogram of water at zero isnaturally or artificially converted into steam, itought to gain, in order to its transformation, andin fact it does gain from surrounding bodies, 535°of heat. These degrees, as can never be too oftenrepeated, are restored integrally by the steam to thesurfaces of all sorts on which its ulterior liquefac-tion takes place. Here we have, as I may observein passing, the whole secret of heating by steam.This ingenious process is very imperfectly under-stood, when it is imagined that the aqueous gasconveys to a distance, in the tubes through whichit circulates, only sensible or thermometric heat;the chief effects produced are owing to the compo-sition-heat, the concealed heat, the latent heat,which is disengaged at the instant that the contactof cold surfaces reconverts the steam from thegaseous to the liquid state.
We may now, then, rank heat among the con-stituent elements of steam. To obtain heat, wemust burn some kind of fuel; steam has, then, acommercial value greater than that of the liquid,by the whole price of the combustible employedin the act of conversion into steam. If the differ-ence between these two values is very great, itmust be attributed principally to latent heat; ther-