172 Mr. "W. J. M. Rankine on the Mechanical Action of Heat. 



had requested him in 1842 to send me for examination some of 

 those remarkable substances. 



Beheve me to be, 



Ever most truly yours, 

 St. Leonard's College, St. Andrew's, D. BrewstER. 



January 20, 1854. 



Note.-'— In using the expression " discovered " I followed the 

 common practice of making a diseoveiy date from its publication ; 

 it would have been more exact to have said " announced the 

 discovery of.^' 



Sir David Brewster's announcement of the discovery was 

 referred by me to the date of the meeting of the British Associa- 

 tion at Southampton, because I was not aware of the previous 

 communication made to the Literary and Philosophical Society 

 of St. Andrew's, nor could I well have been. I shall therefore, 

 I hope, be held excused for not having alluded to the latter. 



G. G. Stokes. 



XXVI. On the Mechanical Action of Heat. By William John 



Macquorn Rankine, C.E., F.R.SS. Land, and Edinb. ^-c. 



[Continued frcn p. 122.] 



Section IV. Of the Mechanical Action of Steam, treated as a 

 Perfect Gas, and the Power of the Steam-engine. 



(21.) TN the present limited state of our experimental know- 

 J- ledge of the density of steam at pressures differing much 

 from that of the atmosphere, it is desirable to ascertain whether any 

 material error is likely to arise from treating it as a perfect gas. 

 For this purpose the ratio of the volume of steam at 100° Centi- 

 grade, imder the pressure of one atmosphere, to that of the 

 water which produces it at 4°*1 Centigrade, as calculated theo- 

 retically on the supposition of steam being a perfect gas, is to 

 be compared with the actual ratio*. 



The weight of one volume of water at 4°*1 Centigrade being 

 taken as unity, that of half a volume of oxygen at 0° Centigrade, 

 under the pressure of one atmosphere, according to the experi- 

 ments of M. Regnault, is 



0-000714900 

 That of one volume of hydrogen . 0-000089578 . (36) 



The sum being 0-000804478 



The reciprocal of this sumbeing multiplied by .. = 1'3641G6 

 * The actual ratio is only known approximately. 



