84 Relations between Chemical Change, [January, 



I may add that this law is proved mathematically by some 

 of the con siderations given above, and confirmed by comparing 

 various experiments of M. Favre with others of M. Raoult. 

 Curiously enough M. Favre gives only the heat which is found 

 by experiments to be evolved in a battery, while M. Raoult 

 only gives that evolved in the exterior circuit. Putting the 

 two together, the truth of the above law becomes abundantly 

 confirmed. 



Chapter II. 



1. What is the mechanical equivalent of heat ? That is 

 to say, what weight will be raised a metre (or foot, or any 

 other unit of length) by the heat which will raise an unit of 

 weight of water from the temperature o° to i° C, and, vice 

 versa, what mechanical energy will produce this amount of 

 heat ? Many distinguished physicists fix the number at 

 about 430, taking any unit of weight and a metre as 

 the unit of length; M. de la Boulaye in several papers 

 published in the " Comptes Rendus " argues for about 180, or 

 less than half that number; Weber and Kohlrausch conclude 

 from their experiments on the mechanical value of electric 

 force, that the oxidation of a milligramme of hydrogen, 

 which produces about 34 gramme-units of heat, will raise 

 226,800 kilogrammes through 1000 metres with a constantly 

 accelerated velocity. Of course, with such enormously 

 discrepant results, there must be a great error somewhere. 

 One calculation makes the equivalent of heat about 430, 

 another about 180, another about 6,000,000,000 ! For a 

 milligramme of hydrogen produces about 34 gramme-units 

 of heat, and 226,800 kilogrammes x 1000 metres = 226,800 

 x 1000 x 1000 grammetres, which divided by 34 gives more 

 than 6,000,000,000 grammes raised one metre high by each 

 gramme-unit of heat. I find it stated that Joule himself, 

 the great authority on the subject, has at different times, 

 and judging by different experiments, varied between the 

 numbers of 80 and 1300 grammetres as the heat equivalent. 



2. But, in the first place, it may be as well to inquire 

 whether there is such a thing as a mechanical equivalent of 

 heat. There may be, or there may not, but we venture to say 

 it has never been proved ; and why are we forced to suppose 

 that the same quantity of heat must always produce the 

 same mechanical effect, whether applied by means of the 

 dilatation of different kinds of gases or of solids, or liquids, 

 or in the many other ways in which it can be applied, and 

 vice versa ? That the same amount of fuel produces the same 

 amount of energy, whether it is consumed in the steam 



