279 



lower temperature ; the excess of heat of the originally warmer water 

 having been all employed in the mere act of melting the ice, or having 

 all become insensible or latent, in the new water formed by melting it. 

 And the principle that heat is absorbed or rendered latent in the pro- 

 duction of steam from water, but is given out or set free again when 

 the former is condensed into the latter, is part of the theory of the 

 steam-engine. But because this phraseology suggests a view of the 

 intimate nature of heat, which is at most hypothetical only, it has 

 by many persons been thought better to use the word specific, in- 

 stead of latent; and to speak of the specific heats of bodies in a sense 

 analogous to that in which we speak of their specific gravities, to ex- 

 press only certain known and measurable properties of these bodies, 

 in relation to the unknown principle of heat. And thus we say, that 

 water has a greater specific heat than mercury, implying only that, 

 whatever be the reason, any given bulk or weight of water produces 

 a more powerful heating effect than is produced by the same bulk 

 or weight of mercury, when both are cooled through the same number 

 of degrees, by contact with a body of a lower temperature. 



The specific heats of solids and of liquids are comparatively easy 

 of determination ; but the great rarity or lightness of the gases 

 renders the measure of their specific heats more difficult. The 

 former may be investigated with much accuracy, by the aid of 

 Laplace's calorimeter: which is an instrument for measuring (by 

 weight) the quantity of ice that is melted by the heat produced or 

 set free in the cooling of a given weight of the proposed solid or 

 liquid body through a given range of temperature. But in applying 

 the same method to the latter question, that is to the inquiry into 

 the heats of the gases, it appears to be difficult to disentangle the 

 small effect of this sort produced by the cooling of any moderate 

 bulk of gas from the effect produced by the cooling of the envelope 

 in which that gas is contained. Several other methods also of inquiry 

 into this delicate subject, however ingeniously devised and carefully 

 executed, by men of deservedly high reputation, have been consi- 

 dered liable to the same or to other objections, and have failed 

 to inspire any general confidence in their results. It seems, how- 

 ever, that the problem has been at length, to a great extent, resolved, 

 by the employment of that other method, which was invented a 



