Principle of the Conservation of Energy. 59 



of different airs and gases, and I then showed how perfectly these 

 experiments proved the truth of the said principle for bodies 

 of that kind. After having obtained this general confirmation 

 of the principle by elastic fluids, I tried to show that the experi- 

 ments of M. (Ersted on the compressibility of (non-elastic) fluids 

 and the heat disengaged thereby were in perfect accordance with 

 my new principle ; and after having shown this, the next step 

 for me was to show that the heat disengaged on the compression 

 of solid bodies also was proportionate to the quantity of mecha- 

 nical energy expended ; and from the experiments of Berthollet, 

 Lagerhjelm, &c. on this subject, I found that here also everything 

 spoke in favour of this principle. 



After, stating these facts, I proceeded to examine what had 

 been done and discovered previously to that time with regard to 

 the heat disengaged by friction, and I found here experiments 

 on a large scale carried out by Rumford, Haldat, Morosi, &c, 

 and I observed with great pleasure that all the results at which 

 these experimenters arrived throughout satisfied the new principle. 

 As these experiments were not made with a view to the deter- 

 mination of the relation between the quantity of heat developed 

 and the mechanical energy expended, it is easy to understand 

 that they were not sufficient to decide the question as to whether 

 the heat disengaged is always proportionate to the mechanical 

 energy lost by friction, and therefore I thought it quite neces- 

 sary to make experiments myself on the quantity of heat deve- 

 loped by friction. 



An apparatus was then constructed by which I was able to 

 measure the quantity of mechanical power lost, and the quan- 

 tity of heat disengaged by friction between 



Brass and Brass, 



Brass and Zinc, 



Brass and Lead, 



Brass and Iron, 



Brass and Wood, and 



Brass and Woollen Cloth, 

 under different pressures and with different velocities ; and the 

 result of all these experiments, nearly 200 in number, was that 

 the heat disengaged was always in proportion to the mechanical 

 energy lost; and if we look more nearly at the figures given in 

 my treatise, which I only presented as a preliminary one, it will 

 be seen that, independently of the materials by which the fric- 

 tion and the heat arose, an amount of mechanical work equal to 

 350 kilogrammetres should be able to raise the temperature of 

 1 kilogramme of water 1° C, which is very near the proportion 

 (365 to 1) that M. Mayer in 1842 supposed, but did not prove, 

 to be right. 



