THE RELATION OF LIFE TO OTHER FORCES. 311 



tion. 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 manifesting itself. 

 It matters not, so that we keep clearly in view the notion 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 incapa- 

 ble, like matter, of being created afresh, except by the Creator. Much 

 of our knowledge on this subject is of course confined to ideas, and gov- 

 erned 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, how- 

 ever, about the relation of one force to another, is not sufficient for the 

 complete destruction of old ideas; and, therefore, in applying the ex- 

 amples of transformation of physical force to the explanation of vital 

 phenomena, we are compelled still to use a vocabulary 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 mani- 

 fested, have been incidentally referred to more than once when describing 

 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 between 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 result 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 specially 

 concerned, resembles somewhat the increase of unorganized matter. The 

 accidental difference of its being in one case superficial, and in the other 

 interstitial, is but little marked in the process as it occurs in the more 

 permanent parts of vegetable tissues. The layers of lignine are in their 



