Southern 07i the Latent Heat of Steam, 119 



be understood to speak when I state my opinion to be as just 

 mentioned ; but it is of that which, when water alters its state 

 to that of an elastic fluid, becomes essential to it in every de- 

 gree of elasticity, besides that which belongs to its expanded 

 state. This latter may be called the latent heat of expa?2sw7i*, 

 while the other may perhaps properly be called constitutional. 

 Allow me here to illustrate hypothetically this matter : If 

 this essential or constitutional part of latent heat be added to 

 water having the necessary portion of sensible heat, and per- 

 fectly confined in a close vessel, I conceive the water would 

 be in the state of an elastic fluid ; would in fact be steam, as 

 dense as water (possibly compressible, and capable of greater 

 density), and would then require no latent heat of expansion ; 

 but if the containing vessel be now conceived to expand, for 

 instance into double the space, I then imagine it would re- 

 quire some addition of heat during this expansion to maintain 

 its proportional elasticity. It must be observed, however, 

 that while this expansion was calling for more latent heat, the 

 sensible heat necessary for the diminishing elasticity would be 

 lessening ; but it does not follow that these quantities should 

 necessarily balance each other f. 



When this fluid, steam, is raised in low temperatures, and 

 of course under a low degree of elasticity, it obtains from its 

 source, at the same instant, not only the constitutional part 

 of its latent heat, but also that of expansion, and thus the two 

 kinds are confounded ; and, in experiments where they are 

 developed by total condensation, are only to be detected to- 

 gether in sum ; and it viay be that this sum, together with the 

 sensible heat, in different states of elasticity, may make a con- 

 stant quantity ; but if the latent heat of expansion from a 

 denser to a rarer state be greater than the diminution of the 

 sensible heat necessary only for the latter, the sum of the sen- 

 sible and total latent heat will be more in steam raised in low 

 temperatures than in high ones, which the result of your ex- 

 periments made in low temperatures seems to countenance. 



In all that I have said above, when speaking of steam, I 

 have always intended that fluid in the state in which it is raised 

 from water, viz. saturated therewith; but undoubtedly this 

 fluid, after it is so raised under any temperature, and being 

 clear from any additional accession of water, may be heated 

 above that temperature, and cooled down to it again with 



• I have no view Iiere to any substances not iiaving the natural power 

 of expansion, as water, ice, &c.— J. S. 



\ 1 have, for very niaay years, entertained a similar hypothesis ; but I 

 know of no experinient whereby the trutli of it can be demonstrated con- 

 clubiveiy."— W. Tliin note by Mr. Wall. 



