398 PEOF. H. L. CALLENDAR, PREFACE TO 



n(n+ l)cP/T in equation (7). It is probable that tbe variation of S with temperatur 

 is not linear, but is more rapid at high temperatures, as indicated by the radiation 

 theory of the variation of specific heat suggested in the Report of the B.A. 

 Committee on Gaseous Explosions (B.A. Rep., 1908, p. 340). At the same time there 

 is probably an appreciable increase over the range 100 C. to 200 C., since the 

 equation of saturation, pressure appears to require a mean value of S = 0'477 over 

 this range, but a lower value from C. to 100 C. 



The observations of KNOBLAUCH and JACOB (' Forsch. Ver. Deut. Ing.,' 35, p. 109, 



190G) extending from 2 to 8 kg./cm 2 and from 150 C. to 350 C., showed a variation 



of S with pressure of the same type as that indicated by equation (7). The absolute 



value of the change with pressure found by these observers agreed with that given 



by formula (7) at a temperature of 210 C., but increased more rapidly below this 



point, mid diminished more rapidly at higher temperatures. On the other hand, the 



value of the specific heat S at xero pressure, obtained by extrapolation, showed an 



increase from 0'447 at 100 (I. to 509 at 400 C., giving nearly the same rate of 



variation between these limits us that found by LAXGEX in his explosion experiments 



at much higher temperatures. This variation could not possibly be reconciled with 



the experiments of HOLBOKN and HENNTNG. A repetition of the observations by 



KNOBLAUCFI and H. MOLLIKU (' Zeit. Ver. Deut, Ing.,' 1911, p. GG5), raised the results 



at 120' C. by 0'02(J, and lowered the results at 400 C. by 0'020, reducing the 



increase with temperature over this range from 0'OG2 to O'OIG, which is quite a 



possible value. The magnitude of the correction, amounting to 10 per cent, on the 



specific heat, illustrates the difficulty of determining the variation of the specific heat 



by the methods which they described, since the variation is of the same order as the 



systematic errors of experiment. This correction brings the results of KNOBLAUCH 



and MOLLIEE into very fair agreement with those given by equation (7), except that 



their results for the variation with pressure near the saturation line, obtained chiefly 



by extrapolation, appear to be excessive, and cannot be reconciled with experiments 



on the cooling effect. 



Although it is probable that there is some variation of S with temperature, the 

 amount of the variation is so small and uncertain over the range required in steam- 

 engine practice that it seems best for the present to take a mean value of S in 

 equation (7) for practical purposes. It is almost certain that the variation of S 

 cannot be linear. No exact theory of the variation has yet been proposed, and 

 any attempt to represent it by an empirical formula would be highly speculative, and 

 would introduce endless complications in the theory, without offering any material 

 advantages so far as the steam-engine is concerned. The mean value of S adopted 

 in 1902 as giving the best general agreement over the required range was 13R/3 or 

 0-478. The present investigation appears to show that the actual value of S at 

 100 C. is G'463, but since it is very difficult to obtain perfectly dry steam near 

 saturation (especially in the throttling experiments), and since S a probably increases 



