THE THEORY OF ENERGY AND THE LIVING WORLD. 517 



imagine a system of bodies inclosed in a tight receptacle and placed 

 upon the pan of a balance, all the chemical reactions which could take 

 place, though they profoundly modified the form and state of the sub- 

 stances, would not be able to affect the equilibrium of the balance. 

 The total weight remains unchanged. It is this equality of weight 

 which is expressed by all the chemical equations. From a more ele- 

 vated point of view we see here the verification of one of the greatest 

 laws of nature, the law of Lavoisier, or the law of the conservation 

 of matter, or, still again, the law of the indestructibility of matter: 

 "Nothing is lost; nothing is created; everything is transformed." 



The conception of energy is not less clear than that of matter; it is 

 only more novel. Its conception requires us to habituate ourselves to 

 the thought that there are no isolated phenomena. The older natural 

 philosophy had but a limited view of things, and considered them as 

 independent of each other. Phenomena were classed for purposes of 

 analysis into distinct groups, such as weight, heat, electricity, magnet- 

 ism, and light. Each phenomenon was considered apart without 

 reference to what preceded or followed after it. Nothing could be more 

 artificial than such a method. In reality every manifestation is linked 

 to some other. There is a metamorphosis from one state of things to 

 another — a mutation. A bond of union connects the state which is 

 anterior to that which follows — the new form which appears with the 

 old form which vanishes. The science of energy shows us that some- 

 thing has passed from the one condition to the other, only covering 

 itself with a new investiture; that in the passage from one state to the 

 other there was something both j)erinanent and active, and that the 

 change is but in aspect. 



The thing which remains constant under the vicissitudes of form, 

 and which connects in a definite manner the antecedent to the phenom- 

 ena which follows, is energy. This gives us, however, but a vague and 

 seemingly arbitrary view of energy. It is to be rendered precise only 

 by the study of examples in mechanical, chemical, thermal, and elec- 

 trical phenomena. Energy takes on corresponding forms throughout 

 these diverse modes. 



Mechanical energy is the most sinrple and the earliest known of these 

 various forms. Mechanical phenomena may be known through two 

 fundamental conditions, space and time, which are of logical origin; 

 and to these is joined a third, which is solely the result of experiment 

 (its origin being in our outward sensation), which is known either as 

 force, as work, or as power. 



Our ideas of force, work, and power have their source in the muscu- 

 lar activity of man. In their definition and development they have 

 employed the genius of the greatest mathematicians from Descartes to 

 Leibnitz. 



A man supports a burden without stooping or bending. It is a 

 weight — that is, a body or mass under the influence of the force of grav- 



