4 Mr. T. R. Edmonds on the Law of Density of 



According to the notation herein used, the quantities P, V, 

 and PV represent ratios of pressure, volume, and expansive force 

 at absolute temperature (a + t) to similar quantities at absolute 

 temperature (a) degrees. That is to say, P, V, and PV are used 



P V PV 



to represent quantities usually represented by p-, ^, and p v , 



the quantities P , V , and P V being constants determined, or 

 assumed to be determined, by observation for the absolute tem- 

 perature (a) degrees. The theoretical numbers contained in the 

 four annexed Tables have been obtained by adopting the abso- 

 lute temperature 376° (corresponding to 100° C.) as the fixed 

 point from which the variable (/) is measured. At this point 

 P =2116"4 pounds to the square foot, as determined by obser- 

 vation. At the same temperature, V = 26*36 cubic feet occupied 

 by one pound weight of water when converted into steam. And 

 at the same temperature, P V = 21 16'4 x 26'36 = 55788 foot- 

 pounds expansive force exerted by one pound weight of saturated 

 steam. These three constants, P , V , and (PV) , may, however, 

 be more conveniently denoted by the letters H, H,, and H /r 



In Table 1. are exhibited the results of twenty out of the 

 twenty-three experiments on the density of saturated steam 

 made by Messrs. Fairbairn and Tate. Three of the experiments 

 made at high pressure and numbered 9', 10', and 14' respect- 

 ively, have been excluded, because the particulars given of these 

 experiments show them to have been defective. Nine of the 

 selected observations were made for temperatures below 100° C; 

 the remaining eleven observations were made for temperatures 

 above 100° C. In the last column of this Table are given, for 

 purposes of comparison, the densities of saturated steam at cor- 

 responding temperatures according to Professor Rankine' s theo- 

 retical Tables as stated by himself in a paper communicated to 

 the Royal Society of Edinburgh in the year 1860. In that 

 paper Professor Rankine expresses his opinion that the results 

 of the experiments of Messrs. Fairbairn and Tate are in near 

 accordance with his own theoretical Tables at all temperatures. 

 He adds, however, that at temperatures above 100° C, the 

 aggregate of experiments indicate an excess of density above 

 the theoretical density. This excess he states to amount on an 

 average to *27 parts of a cubic foot, being the difference between 

 the experimental and theoretical volumes occupied by one pound 

 weight of saturated steam. It will be found on reference to the 

 column of differences in the Table annexed, that the above dis- 

 crepancy has been reduced to one-fifth part of a cubic foot. 

 This has been effected by excluding the three defective experi- 

 ments mentioned, and by correcting an erroneous number given 

 by Professor Rankine. 



