118 Southern on the Latent Heat of Steam. 



was found when the contained water was at 80°, 1° was lost 

 in five minutes; when at 60°, 1° was lost in ten and a half 

 minutes; it would therefore probably lose 1° in eight minutes 

 during the time of an experiment, the mean temperature being 

 about 65°; and as the excess of temperature at the beginning 

 and end of an experiment above that of the air was nearly the 

 same in all three, the loss would be nearly proportional to the 

 duration of each. Hence, to the acquired heat should be 

 added, in the first experiment, 1|°; in the second, ^°; and 

 in the third, |°; being severally proportional to the said du- 

 ration. These being respectively added to the temperatures 

 in column V. and VI., give in the former 81f°, 82° and 8U°; 

 and, in the latter, 33f^, 34-° and SSf"; and if either of these 

 sets of numbers be used in the calculation, according as one 

 or the other of the equations is adopted to develope the re- 

 sults, they will be found to be 1171°, 1212°, and 1 245° for 

 the sums of the latent and sensible heat; and consequently 

 the latent heat in each experiment will be 942°, 942° and 

 950°. 



It may be remarked, that no allowance was made in calcu- 

 lating from the former experiments, for the heat which would 

 be taken by the cistern, but which in the first of them, lasting 

 two hours, would probably be very sensible, and may account 

 for the principal part of the deficiency of latent heat brought 

 out by the calculation from that experinjent, in comparison 

 with that from the two succeeding ones. 



The opinion which I entertain from these experiments as 

 to the latent heat of steam is, that it is a constant quantity, and 

 perhaps this opinion obtains support from the modern dis- 

 coveries of definite proportions. But it is necessary, however, 

 to explain the limitation with which I here use this term, 

 *' co7ista7it quantity." It is well known that if common air be 

 expanded, cold is produced*; and it must therefore happen, 

 that if a given quantity of it at a given temperature could be 

 gradually expanded, and as it was so expanded, gradually 

 supplied with heat, so as to keep the temperature unaltered, 

 this supply of heat would become latent ; the thermometer 

 would not show it. It is probable, both from analogy and 

 experiment, tliat this effect takes place in the expansion of 

 steam. It is not ofthisjiart of its heat, though latent, and in 

 the experiments above related undistinguishable, that I would 



* An opportunity occurred to me some years back, which enabled me 

 to determine, with tolerable precision, the degree of cold produced by the 

 expansion of common air from the bulk of two to three, which I found to 

 be 19° or 20°. 



