*OS HELMHOLTZ 



between the two. His experiments show that when heat is 

 produced by the consumption of work, a definite quantity 

 of work is' required to produce that amount of heat which is 

 known to physicists as the unit of heat; the heat, that is to 

 say, which is necessary to raise one gramme of water 

 through one degree centigrade. The quantity of work 

 necessary for this is, according to Joule's best experiments, 

 equal to the work which a gramme would perform in fall- 

 ing through a height of 425 metres. 



In order to show how closely concordant are his numbers, 

 I will adduce the results of a few series of experiments 

 which he obtained after introducing the latest improve- 

 ments in his methods. 



1. A series of experiments in which water was heated 

 by friction in a brass vessel. In the interior of this vessel 

 a vertical axle provided with sixteen paddles was rotated, 

 the eddies thus produced being broken by a series of pro- 

 jecting barriers, in which parts were cut out large enough 

 for the paddles to pass through. The value of the equiva- 

 lent was 424.9 metres. 



2. Two similar experiments, in which mercury in an iron 

 vessel was substituted for water in a brass one, gave 425 

 and 426.3 metres. 



3. Two series of experiments, in which a conical ring 

 rubbed against another, both surrounded by mercury, gave 

 426.7 and 425.6 metres. 



Exactly the same relations between heat and work were 

 also found in the reverse process that is, when work was 

 produced by heat. In order to execute this process under 

 physical conditions that could be controlled as perfectly as 

 possible, permanent gases and not vapours were used, 

 although the latter are, in practice, more convenient for 

 producing large quantities of work, as in the case of the 

 steam-engine. A gas which is allowed to expand with mod- 

 erate velocity becomes cooled. Joule was the first to show 

 the reason of this cooling. For the gas has, in expanding, 

 to overcome the resistance which the pressure of the atmos- 

 phere and the slowly yielding side of the vessel oppose to 

 it; or, if it cannot of itself overcome this resistance, it sup- 

 ports the arm of the observer which does it Gas thus per- 



