\ll{ \V \ <H\\V ON HYGKOMKTUir .MHTHui-s 97 



temperature a deposit of moisture is formed on the gloss. The observations of 

 l!i:t;NAULT and HKI:\\ n, are in favour of this view; and, if that be so, it is clearly 

 impossible to conduct saturated air along glass tubes to the drying tubes ; some of 

 the moisture will be deposited on the way. How much moisture will be required to 

 saturate the glass is, I believe, unknown ; but if the conducting tubes are maintained 

 at a temperature a degree or more above that of the air when it is being saturated 

 the deposit will not take place.* Thus, in the observations upon air saturated at a 

 temperature below that of the glass, the air could only travel a very short distance 

 before it came in contact with glass at a temperature higher than its own, and the 

 amount of moisture necessary to saturate this short length of glass would l>e supplied 

 by the saturated air passed through the apparatus as a preliminary part of the 

 experiment. So that this suggestion seems to me to explain satisfactorily what is 

 otherwise a great difficulty, namely, that the saturation pressure agrees with the 

 tabulated pressure only if the air is heated after being saturated. 



To assume that the result of the chemical method should agree exactly with that 

 given by the temperature of the saturator is to assume not only DAL/TON'S law to be 

 true, but also that the expansion of vapour with rise of temperature from the 

 saturation point takes place, according to the law of GAV-LusSAC, with a coefficient 

 of expansion the same as that of air. The specific gravity of steam which must be 

 substituted, therefore, in order to obtain identical results by the two methods is not 

 the actual specific gravity of steam in the unsaturated air experimented on, but what 

 would be the specific gravity if the vapour obeyed GAY-LUSSAC'S law in the manner 

 indicated, or (since the specific gravity is referred to air at the same temperature and 

 pressure) the specific gravity of the saturated vapour. Thus the observations may be 

 employed (as RKGNAULT employed similar observations) to determine the specific 

 gravity of saturated steam at the temperature of the saturator. The observations 

 do not admit of sufficient accuracy to trace the variation of the specific gravity with 

 temperature, but the mean specific gravity for the 32 experiments of Table VII. for 

 temperatures of steam between 1 C. and 21 C., if referred to air at the same tem- 

 perature and pressure, is '6245. 



CLAUSIUS has calculated the specific gravity of saturated steam by thermodynamic 

 reasoning from other known constants of steam ('Mechanical Theory of Heat,' p. 153). 

 Assuming the value at C. to be '622, he gives the value at 50 C., '631. From 

 his results, therefore, we obtain '6240 as the mean value between 1 C. and 21 C., 

 a result agreeing very closely with that obtained from my observations. 



If, however, the specific gravity of saturated steam be greater than that of unsatur- 

 ated steam, the vapour cannot obey GAY-LlJSSAc's law in the manner stated, and in 

 consequence the pressure of the vapour of unsaturated steam, calculated by the 

 chemical method, with the true specific gravity of unsaturated steam ('622) ought to be 

 somewhat greater than that given in the table of pressures of saturated vapour ; and 

 the results of Table VII. may accordingly be held to prove that the true pressure of 



* Sec note at end of paper (p. 149). 

 MIXtVLXXXVIII. A. O 



