232 M. Arago's Biographical Memoir of James IFatt. 



much grander scale. At the moment that steam, at the tem- 

 perature of 212°, is produced, the water, at the same tempera- 

 ture of 212°, impregnates itself, — under the form of latent ca- 

 loric, — under a form quite insensible to the thermometer, — 

 with an enormous quantity of heat. Again, when the steam 

 reassumes the liquid state, this caloric of composition is disen- 

 gaged, and goes to heat every thing in its way which is sus- 

 ceptible of absorbing it. If, for example, we were to cause 

 two pounds of steam at 212° to pass through ten pounds of 

 water at the freezing point, the steam would be [wholly lique- 

 fied, and the twelve pounds which would result from the mix- 

 ture would be found at the temperature of 212°. Into the 

 intimate composition of two pounds of steam, there enters 

 therefore a quantity of latent caloric sufficient to raise two 

 pounds of water, whose evaporation is prevented, from the 

 freezing point to the height of 995° Falir. This result will 

 without doubt appear enormous, but it is quite certain. Steam 

 exists only upon this condition. Wherever two pounds of 

 water at the freezing point are evaporated, whether naturally 

 or artificially, in undergoing the transformation, they must 

 seize upon, and in fact do seize upon, 995° of caloric derived 

 from surrounding objects. This number of degrees (for it 

 cannot be too often repeated), the steam entirely restores to 

 the surfaces of whatever nature, upon which the condensation 

 is ulteriorly effected. And here, we may remark in passing, 

 is the whole secret connected with the art of heating by steam. 

 That individual would have a very erroneous idea of this in- 

 genious contrivance, who supposed that the steam conveyed 

 to a distance in the tubes in which it circulates, nothing more 

 than sensible or thermometric heat. The chief effects are, 

 beyond all doubt, owing to the caloric of composition — the 

 hidden or latent heat — which is disengaged at the moment 

 when the contact of the cold surfaces converts the vapour from 

 the gaseous into the liquid state. We must therefore rank 

 caloric among the constituent principles of steam. Caloric 

 is obtained only by the combustion of wood, coal, &c. Steam 

 therefore has a commercial value superior to that of the 

 liquid, by the whole price of the combustible employed in the 

 process of vaporization. If the difference of these two values 



