On the Steam Calorimeter* 



221 



considerable deduction from the increase of weight indicated by 

 the balance. It is to be remembered that the density of steam at 

 100°C. is about half that of air at 10°0. The effect on the apparent 

 weight of the substance will in fact be observable even if it displace 

 a volume of but one cubic centimetre, and the deduction becomes 

 very necessary when dealing with bulky substances. 



The following table contains the density of saturated steam over 

 the range of barometric variation. It is calculated from the formula 

 of Zeuner,* 



y = up 1 ' 11 , 



in which, when p is expressed in atmospheres, a has the value 0'6061, 

 l/» the value 0'9393. 



Table II. — Mass of a Cubic Centimetre of Saturated Steam in Grams. 



Pressure 

 in mm. 



Grams. 



Pressure 

 in mm. 



Grams. 



Pressure 

 in mm. 



Grams. 



730 



•000583 



750 



•000598 



770 



•000613 



735 



•000587 



755 



•000602 



775 



•000617 



740 



•000591 



760 



•000606 1 



780 



•000621 



745 



•000594 



765 



•000610 



785 



•000625 



The results obained from this formula and embodied in the table 

 agree well with deductions based on Regnault's experiments on the 

 total heat of steam. The rate of variation with rise of temperature 

 is closely represented by the formula, but it is noteworthy that the 

 values themselves depart somewhat from Fairbairn's and Tate's expe- 

 rimentally found values. f Thus, according to the latter observers, 

 the density at 100° is 00006187. JSTow, although in general a small 

 error in those values is not of great import — an error of as much as 

 5 per cent, would most generally have an inappreciable effect on the 

 estimation of w — yet cases may arise when a close value is desirable. 



In the hopes of deciding between the various values assigned to 

 the density of steam, I made some direct experiments in the calori- 

 meter. These, in fact, became necessary in the course of some early 

 experiments on the specific heat of air at constant volume, when the 

 displacement difference of a spherical copper vessel having a volume 

 of about 164 c.c. had to be considered. Although the experiments 

 are not as concordant as could be desired, their object is, I think, 

 sufficiently attained. I, therefore, add a short account of them here. 



* ' Theorie Mecanique de la Chaleur,' p. 286. 



f < Phil. Trans.,' vol. 150, I860, p. 185 ; vol. 152, 1862, p. 591. 



