56 THE NATURE AND [SECT. n. 



Having followed through the best information hitherto laid before the public, on 

 the heat required to produce steam, our next object must be to convert it into a 

 form more directly useful for our purpose ; for the quantity of heat which converts 

 a liquid into vapour, requires the additional facts of the volume of the vapour, 

 and its elastic force, to render it valuable. 



OF THE ELASTIC FORCE OF STEAM. 



83. To obtain a rule for determining the force of steam at any temperature, or 

 the temperature corresponding to any given force, we must have recourse to a rule 

 found by trial from the best experiments : it is not a satisfactory method, but we 

 have no other means of arriving at a rule in a case where the real causes of vari- 

 ation are not understood. We still, however, may gain some assistance, from pre- 

 vious reasoning, in forming our conclusions. In the first place, the index of the 

 power representing the law of variation must be of such a simple kind as to render 

 it probable that it is the true one. Hence the index 5'13 employed by Mr. 

 Southern l is not likely to represent the law of nature : Mr. Creighton's index 6, 2 

 or Dr. Young's, which is 7, s are either of them more likely to be accurate. The 

 true equation may be very complex, but this is not probable, and while we are 

 ignorant of its nature, and can represent the results sufficiently near for practical 

 use, by one index, it is best to adopt the simplest form, and particularly when it is 

 equally as likely to be the true one as one of a more complex kind. In any 

 attempt to find the index by the usual method of differences, the errors of expe- 

 riment will have too great an influence. 



84. Secondly. It appears probable that there is a degree of cold at which 

 steam cannot exist ; * and this must be the case when it is condensed by cold, till 

 the cohesive attraction of the particles exceeds the repellent force of the caloric 

 interposed between them ; and the change from an elastic fluid to a solid may then 

 take place without the intermediate stage of liquidity. This physical circumstance 

 enables us to fix another element in the calculation ; for there must be a tem- 

 perature when the force is nothing. 



85. Thirdly. The greatest possible force of steam must next be considered ; 

 for we are certain that our formula must be in error if it exceeds that limit. 

 Suppose a given quantity of water, a cubic inch for example, to be confined in a 



1 Robison's Mechanical Phil. vol. ii. p. 172. - Phil. Mag. vol. liii. p. 266. 



3 Natural Phil. vol. ii. p. 400. 



* An interesting paper on this subject by Mr. Faraday renders it equally so, and shows that 

 the limit is different for different vapours : my formula had led me to the same conclusion ; 

 hence it has another property, justified by experience. See Phil. Mag. vol. kviii. p. 344. 



