438 



passing in, is, adopting the Newtonian law, proportional to 

 the rise of temperature at such instant. But the gas having 

 been always introduced in my experiments at a uniform rate, 

 the rise of temperature is proportional to the time. Hence 

 the velocity of cooling at any instant is proportional to the 

 time. Such being the case, the well-known theorems, which 

 relate to the motion of a material point actuated by a constant 

 force, are here strictly applicable ; and, amongst the rest, that 

 the space (number of degrees) through which the cylinder 

 cools in the time m, is equal to half the rectangle under the 

 time and the last acquired velocity. This theorem, in fact, 

 immediately gives the correction in question, not, I may ob- 

 serve, in an approximate, but in a complete manner, and, in 

 practice, 1 have every reason to be satisfied with it. 



" In what precedes it will be seen, that I have employed 

 the Newtonian law of cooling, which the researches of Dulong 

 and Petit have shown not to represent observations with rigour, 

 except when the excesses of temperature are small. My results, 

 however, are not on this account appreciably less accurate, for 

 the thermometer which I employed only read to tenths, and 

 the divergence of the Newtonian law from the truth, within 

 the range of my experiments, is only observable in the second 

 decimal place. 



" Having explained every thing necessary to enable the 

 Academy to judge of the accuracy of my results, 1 shall now 

 state the numbers at which I have arrived : 



Equal weights. An atom. 



Ammoniacal gas passed into water, 940° 940° 



Muriatic acid gas passed into water, 885° 1900° 



Weight for weight, then, ammonia gives out more heat than mu- 

 riatic acid ; but an atom of the latter gives out almost exactly 

 the double of the heat evolved by an atom of the former. 



" The number for ammonia, it will have been observed, 

 does not materially differ from that for aqueous vapour of maxi- 

 mum density at 212°, the latter having been fixed, by the 



