458 BIOLOGY. [BOOK vn. 



becomes clear if we substitute the word movement, which has a 

 clear and definite meaning, for the word force, which is nebulous 

 and metaphysical. . 



Movement is essential to matter, and the atoms of all bodies, 

 as well those of the impalpable ether as those of our hardest 

 metals filling space, are animated with violent and varied move- 

 ments, which, according to .the modes of their rapidity, direction, 

 amplitude, &c., &c., produce upon our senses diverse impressions. 

 But as in their origin these movements are essentially analogous, 

 they are very easily transformed into each other, since for that 

 purpose it is sufficient for them to change their type. 



We must here recall the principal points of this great doctrine 

 of the unity of forces, founded by Dr. Mayer, and which has 

 totally revolutionised physics. 



The English natural philosopher Joule has demonstrated that 

 every body undergoing mechanical violence, friction, a shock, 

 &c. , grows warm in proportion to the gravity of labour expended, 

 that is to say that the movement of totality which has shaken 

 its mass is then entirely transformed into calorific molecular 

 vibrations. We can then determine with exactitude the mechani- 

 cal equivalent of heat, that is to say, find how many kilometres 

 are necesary to raise by 1 degree the temperature of 1 kilo- 

 gramme of water. All know that the kilogrammetre represents 

 the labour necessary to raise a weight of 1 kilogramme to a 

 height of 1 metre. Joule found that 424 kilogrammetres were 

 necessary to obtain this result. The mechanical equivalent of 

 heat in relation to water is, then, according to this calculation, 

 424. Reciprocally, 1 kilogramme of water, falling freely from a 

 height of 424 metres, and abruptly arrested in its fall, will give 

 1 degree Centigrade of temperature. 



Furthermore, the amount of heat necessary to raise by 1 

 degree the temperature of 1 kilogramme of water has been 

 called calory. 



The calory is, then, about equivalent to 424 kilogrammetres. 



But it is easy to show by very numerous experiments that the 



