fr&m the Cold induced hy the Evaporation of Water. 109 



ever intensity, is understood to be constant during the time of 

 observation. The Doctor also employs a formula to reduce the 

 force of vapour, at the dew-point, to the force which it has in 

 the air, at the actual temperature, although these, being under 

 the same pressure, and each of them forming the same propor- 

 tion of it, are precisely equal, and require no reduction what- 

 ever. — (Ibid, p. 581 ; and in this Journal, vol. xiii. p. 226, first 

 series). Neither Mr Dalton nor Mr Daniell ever think of using 

 any such reduction. 



It has become a standard doctrine in almost every scientific 

 compilation, that a given weight of aqueous vapour or steam, in 

 a state of saturation, contains the same quantity of heat, what- 

 ever be its volume or temperature. But a doctrine'*s being ge- 

 nerally received, is not always a proof of its soundness; and so 

 it happens in the present instance : for, that a given weight of 

 saturated steam really contains more heat at a lower temperature, 

 and consequently under a larger volume, than at a higher tem- 

 perature, and of course under a smaller volume, appears from 

 this : — If we open a cock in the cover of a steam-boiler, when 

 the pressure within amounts to only one atmosphere, the vapour 

 which issues preserves its transparency till it get to some dis- 

 tance from the orifice •, but when the pressure within amounts to 

 two or three atmospheres, the steam issues cloudy and opaque 

 from the very orifice. This clearly shows that the same quan- 

 tity of heat which had been amply sufficient 1o maintain the 

 steam in the elastic^ and consequently transparent form, within 

 the boiler, is quite inadequate to do so under a larger volume 

 and lower temperature. M. Clement found that, on condensing 

 three equal weights of steam, having the unequal forces of one, 

 two, and three atmospheres respectively, in three equal quanti- 

 ties of cold water, the rises of temperature were equal ; and it is 

 upon this complex and fallacious experiment that the doctrine I 

 have questioned has been founded. It is, however, to be ob- 

 served, that when the stream of steam is very small, as it must 

 have been in his experiments, it has, while in the act of dilating 

 from under a pressure of two or three, to that of one atmosphere, 

 the opportunity of absorbing heat from the metal of the very 

 slightly open stop-cock and pipe, then much hotter than 212*, 

 the temperature to which the dilating is supposed to reduce the 



