INTROD.] HYPOTHESES. 3 



combine together only in certain quantities, or in multiples of them; 

 that each body has its proper combining quantity, and that it 

 never enters into combination except in that quantity, or some 

 multiple of it. This is an ultimate fact, ascertained by numerous 

 experiments, and indicates the law, which is so important in che- 

 mistry, that bodies unite with each other in their combining pro- 

 portions only, or in multiples of them, and in no intermediate 

 proportions. And this, again, has led to the beautiful generaliza- 

 tion of Dalton, that the ultimate atoms of bodies are their respective 

 combining quantities, and bear to each other the same proportion 

 as their combining equivalents do. 



Or, to take an example from the science which is to form the 

 subject of the following pages. The function of respiration in 

 animals is a very complex process, respecting the nature of 

 which many unsatisfactory hypotheses had been formed, owing 

 to the obscurity in which many of the phenomena, immediately 

 or remotely connected with it, were involved. Until the law of 

 the diffusion of gases, and of the permeability of membranes by 

 them, had been developed, and until it had been shewn that car- 

 bonic acid is held in solution in venous blood, no theory of respira- 

 tion could be framed adequate to explain all the phenomena. It is 

 now proved, that, in this process, a true interchange of gases takes 

 place through the coats of the pulmonary blood-vessels, the oxygen 

 of the air abstracting and occupying the place of the carbonic acid 

 of the blood. An admirable example is thus afforded of a most impor- 

 tant vital process taking place in obedience to a purely physical law. 



Living objects are those which properly belong to the science of 

 Physiology. These are strongly contrasted with the inanimate 

 bodies (which have never lived), to which other branches of natural 

 science refer. At the same time, there are many points of resem- 

 blance between them ; and as both owe their origin to the same 

 Creative mandate, and are reducible (as will be seen by-and-by) 

 to the same elementary constituents, so they are subject in a great 

 degree to the same physical laws, and are to be investigated accord- 

 ing to the same principles of philosophical inquiry. 



We propose, in the first place, to compare living or organized 

 bodies, with inanimate, mineral, or unorganized bodies, and to ex- 

 plain what is meant by the term Life. Secondly, to review briefly, 

 and with reference to their leading distinctions, the phenomena of 

 the vegetable and animal kingdoms. Thirdly, to point out the value 

 of a knowledge of Physiology, especially that of Man, in relation to 

 medicine, and to explain the best mode of pursuing it. 



B2 



