246 METAMORPHOSIS OF TISSUE. 



have been established, and are highly useful in investigation, are 

 utterly devoid of any theoretical basis ; for many methods have 

 been sanctioned by chemical use which would not stand the test 

 of a logical inquiry. Thus, for instance, no one hesitates to 

 employ predisposing affinity as a means of explanation, although 

 this is nothing more than the personification of an obscure idea. 

 Does not affinity in the mass contradict the fundamental idea of 

 chemical affinity ? We do not speak of the theory of the organic 

 radicals, for the constant alterations and the uninterrupted 

 modifications to which this theory has been subjected, sufficiently 

 attest the slight degree of stability which it possesses. And has 

 not the most prolific of all new theories, by which chemical science 

 has been enriched to such an extraordinary extent with the most 

 important facts the theory of the conjugated compounds, not- 

 withstanding the noble experiments and the brilliant discoveries 

 to which it has led been in turn subjected to every form of 

 modification ? 



We must therefore never forget, in applying our chemical 

 ideas to the elucidation of vital phenomena, that the basis on 

 which they are reared is far less firmly established than the 

 fundamental propositions of physics. We find that even in 

 physics new observations and discoveries are daily being made, 

 which long continue to excite our wonder before we are able to 

 reduce them to known physical principles. How many futile 

 attempts have been made to explain Leidenfrost's experiments! 

 And are there not many even at the present day who regard with 

 wonder the experiments of Boutigiiy ? Is the doctrine of 

 molecular attractions so well developed in physics as to preclude 

 the possibility of being further called in question ? It is only in 

 the present day that we have had a direct proof of the motion of 

 the earth afforded us by Foucault. Yet how far is chemistry 

 behind physics in its fundamental principles ! In endeavouring, 

 therefore, to decide questions of physiology in chemical modes of 

 expression, we cannot exercise too great caution in our deductions ; 

 for we know but too well that in most cases we are only sup- 

 porting one hypothesis by means of another, and that truth in 

 chemistry is very often little more than an idea embodied in a 

 systematic form. 



When, mindful of our fallibility, we once more review the 

 character of the substances which nature employs to produce the 

 most varied effects in the living organism, and to realise the most 

 multifarious purposes, we are struck here, as everywhere, by the 



