and the Laws regarding the Nature of Heat, 115 



and it is less to be wondered at if the same result should occur 

 with the vapour at its maximum density. 



If, on the contrary, the real coefficient of expansion for the 

 vapour were sought, that is to say, the number which expresses 

 the expansion of a certain quantity of vapour taken at a definite 

 temperature and in a state of maximum density, and heated under 

 a constant pressure, we should certainly obtain a value greater, 

 and perhaps considerably greater, than 0-003665. 



From the equation (26.) the relative volumes of a unit weight 

 of steam at its maximum density for the different temperatures, 

 as referred to the volume at a fixed temperature, is readily esti- 

 mated. To calculate from these the absolute volumes with suffi- 

 cient exactitude, the value of the constant A must be established 

 with greater certainty than is at present the case. 



The question now occurs, whether a single volume may not 

 be accurately estimated in some other manner, so as to enable 

 us to infer the absolute values of the remaining volumes from their 

 relative values. Already, indeed, have various attempts been made 

 to determine the specific weight of water vapour ; but I believe 

 for the case in hand, where the vapour is at its maximum den- 

 sity, the results are not yet decisive. The numbers usually given, 

 particularly that found by Gay-Lussac, 0'6235, agree pretty well 

 with the theoretic value obtained from the assumption, that two 

 measures of hydrogen and one of oxygen give by their combina- 

 tion two measures of vapour, that is to say, with the value 



2x0-06926 + 1-10563 ^q.^qq 



These numbers, however, refer to observations made, not at those 

 temperatures where the pressure used was equal to the maximum 

 expansive force, but at higher ones. In this state the vapour 

 might nearly agree wath the law of M. and G., and hence may 

 be explained the coincidence of experiment with the theoretic 

 values. To make this, however, the basis from which, by appli- 

 cation of the above law, the condition of the vapour at its max- 

 imum density might be inferred, would contradict the results 

 before obtained ; as in Table IV. it is shown that the divergence 

 at the temperatures to which these determinations refer are too 

 considerable. It is also a fact, that those experiments where the 

 vapour at its maximum density was observed have in most cases 

 given larger numbers; and Regnault* has convinced himself, 

 that even at a temperature a little above 30°, when the vapour 

 was developed in vacuo, a satisfactory coincidence was first ob- 

 seiTed when the tension of the vapour was 0*8 of that which 

 corresponded to the maximum density due to the temperature 

 * Ann. de Chim. et de Phys., 3 ser. vol. xv, p. 148. 



