CHEMICAL AFFINITY. 151 



organic chemistry. As, in the case of cyanogen, a body 

 obviously compound discharged in almost all its reactions the 

 functions of an element, so in many other cases it was found 

 that compound bodies, in which a number of elements existed, 

 might be regarded as binary combinations, by considering 

 certain groups of these elements as a compound radical ; that 

 is, as a simple body when treated of in relation to the other 

 complex substances of which it forms part, and only as non- 

 elementary when referred to its own internal constitution. 



Undoubtedly, by approximating in theory the reactions of 

 inorganic and of organic chemistry, by keeping the mind 

 within the limits of a beaten path, instead of allowing it to 

 wander through a maze of isolated facts, the doctrine of com- 

 pound radicals has been of service ; but, on the other hand, 

 the indefinite variety of changes which may be rung upon the 

 composition of an organic substance, by different associations 

 of its primary elements, makes the binary constituents vary 

 as the "minds of the authors who treat of them, and makes 

 their grouping depend entirely upon the strength of the ana- 

 logies presented to each individual mind. From this cause, 

 and from the extreme license which has been taken in 

 theoretic groupings deduced from this doctrine, a serious 

 question arises whether it may not ultimately, unless carefully 

 restricted, produce confusion rather than simplicity, and be to 

 the student an embarrassment rather than an assistance. 



The term 'law' has not unfrequently been applied to 

 groupings of phenomena such as those to which I have been re- 

 ferring ; it seems to me that a popular misconception as to the 

 meaning of the term prevails. Putting aside what are called 

 necessary truths, such as those proved by mathematics, and 

 the vexed metaphysical question as to whether these are or 

 are not independent of experience, we can, I think, have little 

 doubt that a vast number of those formulae to which the 

 term ' laws ' are commonly applied are merely categories into 

 which our own experience has thrown phenomenal instances. 

 The so-called law of chemical equivalents, the laws of refrac- 

 tion and polarisation of light, of the reciprocal action and re- 

 action of electricity and magnetism, nay, the very grouping 

 of phenomena under the terms electricity, magnetism, &c., 



