190 P el it; and Dulon g on some important Points [Sept. 



of the labours of many philosophers, who have extended to a 

 great number of bodies the methods which they have either 

 contrived or improved. Several of them have likewise endea- 

 deavoured to confirm, by their own experiments, some conse- 

 quences deduced from the notions winch they have formed to 

 themselves of the nature of heat, and of its mode of existence in 

 bodies. Accordingly Irvine and Crawford, admitting that the 

 quantity of heat contained in bodies is proportional to their 

 capacity, have concluded, that whenever the specific heat of a 

 compound is greater or less than the sum of the specific heats of 

 its elements, there ought to take place at the instant of combina- 

 tion either an absorption or disengagement of heat ; but this 

 principle, which Irvine had already applied to the circumstances 

 which accompany the changes in the state of aggregation, and 

 which Crawford made the basis of his theory of animal heat, is 

 in opposition with too many facts to be adopted. The same is 

 the case with a very ingenious hypothesis proposed by Mr. 

 Dalton. According to the ideas of this celebrated philosopher, 

 the quantities of heat united to the elementary particles of the 

 elastic fluids are the same for each. Hence we may, setting out 

 from our knowledge of the number of particles contained in the 

 same weight or the sanse volume of the different gases, calculate 

 the specific heats of these bodies. This Mr. Dalton has done ; 

 but the numbers which he obtained, and those likewise deduced 

 from several other better founded hypotheses on the constitution 

 of gases, are so inconsistent with experiment that it is impossible 

 for us not to reject the principle upon which such determinations 

 are founded, which Dalton has presented merely in a theoretic 

 manner. The attempts hitherto made to discover some laws in 

 the specific heats of bodies have then been entirely unsuccessful. 

 We shall not be surprized at this if we attend to the great inac- 

 curacy of some of the measurements ; for if we except those of 

 Lavoisier and Laplace (unfortunately very few) and those by 

 Laroche and Berard for elastic fluids, we are forced to admit 

 that the greatest part of the others are extremely inaccurate ; as 

 our own experiments have informed us, and as might indeed be 

 concluded from the great discordance in the results obtained for 

 the same bodies by different experimenters. It is not uncommon, 

 for example, to meet with numbers in the best tables three or 

 four times as great as they ought to be. 



Our first care then was necessarily directed to what could 

 render the measurements that we were to use as accurate as 

 possible. Among the methods of determining the capacities of 

 bodies, those in which the melting of ice or the mixture of bodies 

 with water is employed, may doubtless, when properly conducted, 

 lead to very exact results; butthe greater number of the substances 

 on which it is indispensable to operate can rarely be obtained in 

 sufficient mass to enable us to apply, either of these methods. It 

 was necessary, therefore, to have recourse to a different method. 

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