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



coefficient of logP, at any temperature t, was always repre- 

 sented by 



the exponent -( = 7i-{-l) being equal to the number 2*302585, 

 tc 



which is the hyperbolic logarithm of 10. 



The first and only satisfactory observations made for determi- 

 ning by experiment the density of saturated steam at various 

 temperatures, are those of Messrs. William Fairbairn and Thomas 

 Tate. They are published in the Philosophical Transactions of 

 the year 1860. The difficulties are great in the way of making 

 correct observations on the density of saturated steam in free 

 communication with water. Such steam has not yet been ob- 

 tained in a pure state, there being always an admixture of water 

 with such steam. Part of the water is suspended in the form of 

 cloud or mist, and part is pressed as a film of fluid against the 

 sides of the containing vessel. Messrs. Fairbairn and Tate 

 appear to have overcome the chief impediments to correct obser- 

 vation by the use of their " saturation-gauge." On examining 

 the results obtained, it will be found that the law of progression 

 according to temperature for the density as given by these ob- 

 servations, is not much less regular than the law of progression 

 for pressure as given by the observations of M. Regnault. It 

 will be found that the function of the variable / involved in the 

 law of density is identical with the function of the variable / 

 involved in the law of pressure. The differential coefficient of 



. fv -2' 302585 



log P has already been found to be + a ( 1 -f - ) . It will 



now be found that the differential coefficient of logV is 



(/\— 2*302585 

 1-1 — J . The constant a for pressure at tempe- 



rature 100° C. was found to be + -03580. The new constant 

 «j for volume at the same temperature will be found to be 

 -•03365. 



In the investigation of the law of pressure above referred to, it 

 has been shown (Philosophical Magazine, vol. xxix. p. 179) that 

 for any given interval of temperature, the logarithm of the pres- 

 sure P of saturated steam is a simple function of the logarithm 

 of the expansive force p of a unit weight of a perfectly elastic 

 vapour maintained at a constant volume. The quantity p being 



= 1 + - as.** 1 , it was there shown that 



rf.logP 



d.lo 



P 



= uap- n =:aae- nt ^=.aae- nl ^P. 



