and on Alcoholic Engines. 39 



its avidity to absorb heat from the contiguous metal would be 

 so much the greater. 



Indeed the very friction occasioned by the violent and agi- 

 tated egress through the stopcock, was likely to augment the 

 heat in the emerging vapour, and so much the more as the 

 pressure of the steam was higher. 



I cannot help thinking that many experiments connected 

 with heat are affected by friction, and by violent motion of the 

 fluids employed ; though I am not aware that any allowance 

 has ever been made, or perhaps can with certainty be made, 

 for this source of error*. 



If some of these remarks should appear to be overstretched, 

 yet the amount of the whole surely goes far to corroborate the 

 opinion of Mr. Watt, that the latent heat of steam decreases 

 rapidly as the temperature rises. M. Clement's experiments 

 show, so far as they go, that the decrease in latent heat is at 

 least as great as the increase of temperature ; and that, neg- 

 lecting friction, &c, the expenditure of heat in an engine of a 

 given power, cannot be greater than in the inverse ratio of the 

 temperature increased by 448°. But this ceconomy of heat, 

 as M. Poisson observes, does not sufficiently account for the 

 advantage gained by high-pressure engines, — a strong pre- 

 sumption in favour of Mr. Watt's opinion ; especially consi- 

 dering the many inconveniences under which high-pressure 

 engines work. 



Contrary to what we have just seen, many practical men 

 reckon the expense of heat as the power, whatever be the 

 pressure of the engine. But they also reckon the quantity of 

 water converted into steam to be always proportional to the 

 power of the machine; whereas it is certain, that the density 



* The source of heat accompanying friction is still involved in obscurity. 

 Such as take heat for a species of motion get more easily over friction than 

 any thing else. But is it improbable, that this heat may be nearly allied 

 to electricity, or like it suddenly collected from some distance ? Supposing 

 the absolute heat in bodies to be very great compared with what would 

 carry their temperatures through the whole range of observation, may not 

 much of the heat of friction, as the Marquis de Laplace alleges, be ex- 

 pressed from the surfaces of bodies by their mutual reaction and compres- 

 sion ? But this hypothesis seems best adapted to solids, and would be- 

 sides require a zero of temperature much more remote than where the 

 Marquis places it, at — 448° F. This zero, derived from the assumption 

 that gases contract uniformly to nothing as their temperatures approach it, 

 militates against the more probable opinion that all the gases admit of li- 

 quefaction and solidification. No wonder that many of the other absolute 

 zeros are so ridiculously absurd, when they are computed on the gratui- 

 tous assumption that the specific heats of bodies are exactly proportional 

 to their absolute heats. Many other chemical calculations are founded on 

 the same slippery ground. 



of 



