'TIS HANDBOOK OF PHYSIOLOGY. 



construction of woody tissue. Thus, coal and wood being products of 

 the expenditure of force, must be taken to represent a certain amount 

 of power ; and, according to the law of the correlation of forces, must 

 be capable of yielding, in some shape or other, just so much as was exer- 

 cised in their formation. The amount of force requisite for rending 

 asunder the elements of carbonic acid is exactly that amount which will 

 again be manifested when they clash together again. 



The sun, then, really, is the prime agent in the movement of the 

 steam-engine, as it is indeed in the production of nearly all the power 

 manifested on this globe. In this particular instance, speaking roughly, 

 its light and heat are manifested successively as vital and chemical 

 force in the growth of plants, as heat and light again in the burning 

 fuel, and lastly by the piston and wheels of the engine as motive power. 

 We may use the term transformation of force if we will, or say that 

 throughout the cycle of changes there is but one force variously mani- 

 festing itself. It matters not, so that we keep clearly in view the no- 

 tion that all force, so far at least as our present knowledge extends, is 

 but a representative, it may be in the same form or another, of some 

 previous force, and incapable like matter of being created afresh, except 

 by the Creator. Much of our knowledge on this subject is of course con- 

 fined to ideas, and governed by the words with which we are compelled 

 to express them, rather than to actual things or facts; and probably the 

 term force will soon lose the signification which we now attach to it. 

 What is now known, however, about the relation of one force to another, 

 is not sufficient for the complete destruction of old ideas ; and, there- 

 fore, in applying the examples of transformation of physical force to the 

 explanation of vital phenomena, we are compelled still to use a vocabu- 

 lary which was framed for expressing many notions now obsolete. 



The dependence of the lowest kind of vital existence on external 

 force, and the manner in which this is used as a means whereby life is 

 manifested, have been incidentally referred to more than once when de- 

 scribing the origin of vegetable tissues. The main functions of the 

 vegetable kingdom are construction, and the perpetuation of the race ; 

 and the use which is made of external physical force is more simple than 

 in animals. The transformation indeed which is effected, while much 

 less mysterious than in the latter instance, forms an interesting link be- 

 tween animal and crystalline growth. 



The decomposition of carbonic acid or ammonia by the leaves of 

 plants may be compared to that of water by a galvanic current. In both 

 cases a force is applied through a special material medium, and the re- 

 sult is a separation of the elements of which each compound is formed. 

 On the return of the elements to their original state of union, there will 

 be the return also in some form or other of the force which was used to 

 separate them. Vegetable growth, moreover, with which we are now 



