176 Prof. Thomson on the Dynamical Theory of Heat. 



that it may remain saturated as its temperature rises ; and con- 

 versely, if a quantity of saturated vapour be allowed to expand 

 in a closed vessel, heat must be supplied to it to prevent any 

 part of it from becoming condensed into the liquid form as the 

 temperature of the whole sinks. This very remarkable conclu- 

 sion was first announced by Mr. Rankine, in his paper commu- 

 nicated to this Society on the 4th of February last year. It was 

 discovered independently by Clausius, and published in his paper 

 in Poggendorfi"^s Annalen in the months of April and May of the 

 same year, 



60. It might appear at fii-st sight, that the well-knoVn fact 

 that steam rushing from a high-pressure boiler through a small 

 orifice into the open air does not scald a hand exposed to it*, is 

 inconsistent with the proposition, that steam expanding from a 

 state of saturation must have heat given to it to prevent any part 

 from becoming condensed ; since the steam would scald the hand 

 unless it were dry, and consequently above the boiling-point in 

 temperatm'e. The exjjlanation of this a])parent difficulty, given 

 in a letter which I wrote to Mr. Joule last October, and which 

 has since been published in the Philosophical ]\Jagazine, is, 

 that the steam in rushing through the orifice produces mecha- 

 nical effect which is immediately wasted in fluid friction, and 

 consequently I'econ verted into heat ; so that the issuing steam 

 at the atmospheric pressure would have to part \vith as much 

 heat to convert it into water at the temperature 100° as it would 

 have had to part with to have been condensed at the high pres- 

 sure and then cooled down to 100°, which for a pound of steam 

 initially saturated at the temperature t is, by Reguault's modi- 

 fication of Watt's law, •305(^ — 100°) more heat than a pound 

 of saturated steam at 100° would have to part with to be re- 

 duced to the same state ; and the issuing steam must therefore 

 be above 100° in temperature, and dry. 



[* Note added June 26, 1852. — At present I am inclined to believe that 

 the rapidity of the cuiTent exercises a great influence on the sensation 

 experienced in the circumstances, by .causing the steam to mix \yith the 

 surroimding an- ; for I have found that the hand suffers pain when exposed 

 to the steam issuing from a common kettle, and dried by passing through 

 a copper tube surrounded by red-hot coals or heated })y lamps. But 

 although there may be uncertainty regarding the causes of the different 

 sensations in the different circumstances, I believe there is no reason for 

 doid)ting either the fact of the dr3'ness of the steam issuing from a high- 

 pressure boiler (except when there is "priming" to a considerable extent), 

 or the correctness of the explanation of this fact which I have given in the 

 letter referred to.] 



