4 ANIMAL CHEMISTRY LECTURE I. 



piece of iron, for instance let us consider how varied have been 

 the states of its existence at different times. We know that it 

 has been at rest and in motion ; it has been silent and sonorous ; 

 it has been luminous and obscure, hot and cold, liquid and solid, 

 magnetic and non-magnetic, electrical and non-electric. But 

 throughout all these changes of rest and motion, sound and 

 silence, heat and cold, &c., the individual piece of metal has con- 

 tinued one and the same ; it has been composed throughout of 

 identically the same matter. Now, so long as a body continues to 

 be one and the same body so long, in fact, as its composition re- 

 mains unaltered so long do all the changes which it manifests 

 belong to the province of physics, and not to the province of 

 chemistry. For this piece of iron to undergo a chemical change, 

 it must cease to be a piece of iron, and become some other body 

 rust of iron, or vitriol of iron, or tincture of iron, or Prussian 

 blue, or clot of blood, or some one of many hundred different 

 combinations. Looking, then, to the chemistry of this piece of 

 iron, we have regard to the state of ironstone in which it existed 

 before it became metallic iron, and to the many different non- 

 metallic states in which it may hereafter exist. The dynamical 

 interest of a body has reference to its existence in time, to its past 

 and future variations of state, even more than to its present con- 

 dition. I venture to impress this point particularly on your at- 

 tention, that while chemistry treats of the composition of bodies, 

 it has special reference to their changes in composition. Now, 

 when we consider that every action of the living body, every 

 growth, every waste, every secretion, every movement, and even 

 every thought is attended by, and consequent upon, a change of 

 chemical composition, we perceive, in an instant, how much the 

 future of physiology must depend on the progress of chemical 

 research how only the iatro-chemist, if I may so call him, can 

 ever hope to understand the varied series of actions, healthy and 

 morbid, which are continually taking place in the living organism. 

 The chemistry, then, of any animal tissue of a piece of muscle, 

 for instance, no less than a piece of iron has reference to its 

 origin and metamorphoses. The chemist looks equally to its 



