388 Prof. Thomson on a remarkable property of Steam 



or some heat must be acquired by the steam as it issues from the 

 boiler. The pretended explanation of a corresponding circum- 

 stance connected with the rushing of air from one vessel to 

 another in Gay-Lussac's experiment, on which you have com- 

 mented, is certainly not applicable in this case, since instead 

 of receiving heat from without, the steam must lose a little in 

 passing through the stop-cock or steam-pipe by external ra- 

 diation and convection. There is no possible way in which the 

 heat can be acquired except by the friction of the steam as it 

 rushes through the orifice. Hence I think I am justified in 

 saying that your discovery alone can reconcile Mr. Rankine's 

 discovery with known facts. 



In connexion with this subject it is to be remarked, that if 

 your fundamental principle regarding the convertibility of 

 heat and mechanical effect, adopted also by Mr. Rankine, be 

 true, a quantity of water raised from the freezing-point to any 

 higher temperature, converted into saturated vapour at that 

 temperature, and then allowed to expand through a small ori- 

 fice wasting all its "work" in friction, will, in its expanded 

 state, possess the " total heat" which has been given to it; but, 

 on the contrary, if it be allowed to expand, pushing out a pis- 

 ton against a resisting force, it will in the expanded state pos- 

 sess less than that total heat by the amount corresponding to 

 the mechanical effect developed. If the proposition quoted 

 above of Mr. Rankine's be true, this amount must exceed the 

 amount of deviation from Watt's law measured by Regnault ; 

 and must consequently bear a very considerable ratio to the 

 total heat, instead of being, as I believe all experimenters ex- 

 cept yourself have hitherto considered it to be, quite inappre- 

 ciable. 



In the paragraph following that from which I have quoted, 

 Mr. Rankine remarks, — " There is as yet no experimental 

 proof" of the preceding proposition. " It is true that in the 

 working of non-condensing engines it has been found that the 

 steam which escapes is always at the temperature of saturation 

 corresponding to its pressure, and carries along with it a por- 

 tion of water in the liquid state ; but it is impossible to distin- 

 guish between the water which has been liquefied by the ex- 

 pansion of the steam, and that which has been carried over 

 mechanically from the boiler." The circumstances of the 

 passage of steam through the various parts of a non-conden- 

 sing engine, are certainly very complicated. Even if there were 

 no water "carried over mechanically from the boiler," we could 

 not conclude the truth of Mr. Rankine's proposition from the 

 fact of the steam issuing moist and at 212° from the waste 

 steam-pipe, since this might be accounted for by the external 



