On the Method of Condensation in Calorimetry. 



353 



observation of the remaining quantities. The value of X has been 

 accurately determined by Regnault and recorded for all values of t 2 ° 

 likely to occur. By similar observations, knowing S, X may be the 

 unknown quantity sought for by experiment, and determined from 

 the equation. A substance of known specific heat being in this case 

 immersed in a vapour of unknown latent heat. 



It is evident that of the quantities to be measured the measurement 

 of the value of w alone presents any difficulty. I have found, how- 

 ever, that w is accurately determined by weighing the quantity of 

 vapour condensing on the surface of the substance in its transition 

 of temperature from ^° to t 2 ° ; and the calorimeters described in this 

 paper are in short contrivances permitting the accurate determination 

 of the weight of condensation as it gathers and falls from the surface 

 of a substance suddenly surrounded by an atmosphere of steam. As 

 the process is one principally of weighing, I have called the apparatus 

 a gravimetric calorimeter. 



Up to the present I have only employed the condensation of water- 

 vapour in effecting determinations of specific heat gravimetrically. 

 There are evidently sum cient reasons in favour of its use, chief among 

 these is the accurate knowledge we possess of its latent heat through 

 Regnault's experiments. But the ease with which it may be gene- 

 rated, and its chemical inertness when in presence of the materials of 

 the calorimeter, are also important. The range of temperature too 

 involved is sufficiently considerable, and, for my purpose, allowed of 

 convenient comparison with the data of Regnault and. others on 

 specific heat obtained generally through this same range. By the use 

 of other vapours greater weights of condensation are obtainable, but 

 this is only of consequence when dealing with very small quantities 

 of matter. The variations, however, in the upper limit of temperature 

 thus obtainable might be otherwise important. 



Description of a Gravimetric Calorimeter. 



Whatever vapour be used in this method of calorimetry, two chief 

 conditions must be attended to in its employment : (1) sudden replace- 

 ment of the air surrounding the substance by the vapour to be con- 

 densed ; (2) perfect mechanical conditions permitting the evaluation 

 of the weight condensed while the substance is still surrounded with 

 the vapour. 



To effect the first of these conditions we might evidently either 

 introduce the substance quickly into a chamber filled with and sup- 

 plied with the vapour, or, maintaining the substance throughout in 

 one chamber, replace the' air around it by sudden and rapid inflow of 

 the vapour. In choosing the last we dispense both with much con- 

 trivance and manipulation involved in the first. The error of trans- 



