[ 398 ] 



LI. On the Deportment of Vapour during its Expansion under 

 different circumstances. By R. Clausius*. 



NOT long ago, Mr. Rankiaef and myself J gave utterance 

 almost contemjDoraueously to the proposition, — that when 

 the saturated vapour of water, contained in a vessel impervious 

 to heat, is subjected to compression, it does not remain saturated, 

 but can part with a certain quantity of heat without being pre- 

 cipitated ; and conversely, when, under the same circumstances, 

 the vapour is suffered to expand, to preserve it from precipitation 

 a certain amount of heat must be imparted from without. 



In connexion wath this proposition, Mr. W. Thomson, in a 

 letter to Mr. Joule, refers to the fact " that the hand may be 

 held with impunity in a current of steam issuing from the safety- 

 valve of a higli-pressure boiler §." From this he concludes, that 

 the stream of vapour carries no water along with it, and holds 

 that this conclusion must contradict the above proposition, if 

 the existence of a source of heat from which the vapour shall 

 receive a quantity suflBcient to preserve it from precipitation 

 cannot be established. This source he finds in the friction which 

 takes place during the issue of the steam from the orifice. 



Although Mr. Thomson himself obsei-ves, in the course of his 

 letter, that, according to the mechanical theory of heat, diff'erent 

 states of the vapour are induced by different methods of expan- 

 sion, still in making the remark cited above he does not appear 

 to have taken this circumstance into accomit. He, in fact, ap- 

 plies the proposition to a case, to which, according to its deve- 

 lopment, it is altogether inapplicable. For vapour escaping 

 from a boiler into the air the theory would give a totally different 

 result, which result may with ease be deduced. 



From the innumerable modifications to which the expansion 

 of the steam may be subjected, I will choose three which may 

 be considered the most important, and in which the essential 

 differences exhibit themselves with peculiar clearness. 



We will consider the matter as subjected successively to the 

 two following conditions : — first, that the vapour during its ex- 

 pansion has to overcome a resistance which corresponds to its 

 entire expansive power ; and secondly, that it escapes into the 

 open air, in which case the pressui*e of the atmosphere alone ia 

 opposed to it. We will further consider the two cases embraced 



* Translated from PoggendorfF's Annaleii, vol. Ixxxii. p. 263. 



t Tiausactions of the Royal Society of Edinburgh, vol. xx. part 1. p. 147; 

 and in extract, Pogg. Ann., vol. Ixxxi. p. 172. 



X Pogg. Ann., vol. Ixxix. pp. 'ii&S and 500. [A translation of this valu- 

 able i)aper is in type, and will appear as soon as we can possibly find room 

 for it. — Eds.] 



§ Phil. Mag., vol. xxxvii. p. 387 ; and Pogg. Ann., vol. Ixxxi. p. 477- 



