12 CHEMISTRY AND PHYSICS IN RELATION 



the mode of action of any thing, or cause, which in acting upon organic matter 

 changes its form or composition. 



TYPICAL FORCE AN INDEFINITE IDEA. 



It cannot surely be correct to regard certain vital indications, (as, for instance, 

 the development of the organism from the egg or germ, or the renewal of original 

 forms,) as dependent on a certain typical power in the organism, since this expres- 

 sion is nothing but a mere verbal illustration. 



Henle, at page 129 of his " Rationelle Pathologic," admits that the perpetual 

 typical laws, which he has spoken of, are inadequate to explain how the salaman- 

 der can regenerate a whole limb, while in the kindred frog regeneration is limited 

 to a few tissues, as in the higher animals ; and regards these indications as proving 

 nothing more than the fact that they are such. To comprehend an explanation, 

 presupposes a knowledge of the laws on which it depends, and the comprehension 

 of the law is inseparable from the knowledge of qualitative or quantitative re- 

 lations. 



By way of rough illustration, we may compare the healthy organism in many 

 respects to a large Transatlantic steamboat ; the latter consumes at every moment 

 of its passage oxygen and fuel, which are again given off in the form of carbonic 

 acid, water, soot, or smoke ; it encloses sources of heat and power, which call 

 forth motor effects, and minister to the wants of the crew, by preparing food for 

 their use. If a sail be rent, there is one at hand to repair it ; if a leak be sprung, 

 the joiner is there to arrest the damage ; while a number of men are ever active in 

 keeping up the original condition of the vessel, and maintaining her speed ; and so 

 it is with the living body, which likewise has its smiths, and joiners, and other 

 artificers. Let it then be our duty to study and recognise its mutual relations. 



LIGHT CONSIDERED AS AN IRRITANT. 



It is impossible to arrive at the comprehension of a subject, if, as is done by 

 some pathologists, a term such as an irritant be made to include alike active 

 causes, which change the form and composition of organic bodies, and such as 

 light, sound, &c., which do not possess this capacity. Light is in itself a motor 

 appearance, and as such is perceived by the eye, exciting in the optic nerve a 

 motion which is transferred to the sensorium ; the motion once begun is continued, 

 as the tones of a flute are prolonged in the air, or a string in the piano produces 

 tones. The impression of light is motion itself, but this motion calls forth no 

 change in the form and composition of the eye or brain, unless new causes are 

 superadded ; and among such we may rank the labor of thought, by which the 

 impression is converted to a conscious perception, awakening, in its turn, concep- 

 tions and ideas. 



No one would seriously maintain that a piece of white paper could, by its re- 

 flected light, bring about a change in the form and composition of the brain, since 

 an opposite effect must then necessarily be ascribed to a piece of black paper, 

 from which no light is given forth ; but the two combined, the black and white, 

 when in the form of letters in a book, awaken the most manifold feelings, concep- 

 tions, and images ; and it is by means of these, and not of light, that an influence 

 is exercised upon the properties of the brain. 



SOUND AS AN IRRITANT. 



The observations which we have made regarding light, apply in every respect 



