304 X>r. Thomson on the Influence of Humidity [April, 



The inspection of this table, which the reader may easily 

 extend ad libitum^ will enable us to explain several phenomena, 

 which, though they have been long known, have not yet, so far 

 as 1 know, been satisfactorily accounted for. Mr. Watt, for 

 example, found, that water could be distilled over in a vacuum 

 very well at the temperature of 70° ; but to his great astonish- 

 ment, the latent heat of the vapour was just as much greater than 

 the latent heat of steam at 212°, as the temperature of 70° was 

 lower than 212°. Now, from the preceding law, it is obvious 

 that this must be the case, and that more fuel is required to 

 distil water in vacuo than in the open air. One of the advan- 

 tages which Mr. Woulfe stated as belonging to his high 

 pressure engines was, that they performed more work with a 

 less expence of fuel than the ordinary steam engines. And I 

 have been told by more than one Cornish gentleman conversant 

 with these engines, that they really save a considerable quantity 

 of fuel. Now it is easy to see from the preceding table taken in 

 conjunction with the known increase of the elasticity of the 

 steam at high temperatures, that this must be the case. The 

 elasticity of steam at the temperature of 344° is eight times 

 greater than at 212° ; while, at the same time, the latent heat is 

 132° less. It is necessary indeed to raise the sensible heat of 

 the water to be converted into steam 132° higher than 212^. 

 But this is an expenditure of fuel only made once for all ; for 

 the water, when once heated to that temperature, may be kept 

 at it with comparatively little fuel. 



Thus steam is employed with the greater economy the higher 

 the temperature to which it is raised. But the great strength 

 necessary for vessels containing high pressure steam, and the 

 greater hability of these vessels to be injured, necessarily sets a 

 limit to the temperature to which the steam can be raised. 



This law, to which the latent heat of vapour is subjected, has 

 struck several persons to whom I have stated it with surprise ; 

 yet it is perfectly analogous to what takes place in other bodies. 

 Thus it is well known that the specific heat of common air 

 increases in proportion to its expansion. This is the reason 

 why the temperature of the air diminishes in proportion as we 

 ascend in the atmosphere. Now the latent heat of vapour is 

 analogous to the specific heat of air. It ought, therefore, to 

 increase in proportion as the particles of the vapour get further 

 and further from each other. We have only to admit that the 

 specific gravity of vapour increases with the elasticity or the 

 temperature, to render the whole perfectly perspicuous. Now 

 every thing conspires to satisfy us that this is really the case ; 

 but if we admit it, we can easily ascertain the specific gravity of 

 Tapour at every temperature. From the experiments of M. Gay- 

 Lussac, it follows that the specific gravity of steam at 212° is 

 0*625, supposing the specific gravity of air at that temperature to 

 be unity. From this, it is obvious, that if we reckon the specific 



