On the Calculus of Chemical Operations. 419 



time, this notation did not assume at first the form it has since 

 acquired : it has been perfected, modified, almost transformed; 

 it is, however, from Gerhardt that it dates, and to him the 

 honour of it is due. Similarly Sir B. C. Brodie makes a bold 

 attempt on a new path which may lead to great results; and 

 we must beware of rejecting what he brings on the ground 

 that it is incomplete. The mathematical analysis proposed by 

 him is as yet but a germ; worked out and developed, it may 

 become an organism. 



This said, and all reservations made as much in favour of 

 as against the new method, we may say at once that the ex- 

 isting atomic notation may be divided into two parts : — that 

 which is entirely hypothetical and metaphysical, and which 

 explains phenomena by the grouping of atoms ; and that which, 

 in spite of the words atom and molecule which offend Sir B. C. 

 Brodie, is not more hypothetical than the notation of Sir 

 B. C. Brodie himself. The whole is, in fact, a matter of de- 

 finition. How do we define a molecule ? It is the smallest 

 portion of matter at which we can arrive by physical division, 

 and of which the weight is equal to two volumes of the vapour 

 considered in relation to the weight of one volume of hydrogen, 

 both at the normal conditions of temperature and pressure. 

 How do we define an atom ? It is the smallest quantity of a 

 given portion of matter attainable by chemical division, which 

 is not subdivided in any operation, and which is always trans- 

 ported integrally from one combination to another. A mole- 

 cule, then, setting aside all metaphysical senses, is the weight 

 of two volumes of a gas or a vapour ; an atom is the weight 

 of two, of one, or of half a volume of a gas or a vapour, ac- 

 cording as, in chemical reactions, the molecule is transported 

 intact or is subdivided into smaller weights. 



Now what does a " simple weight " represent for Sir B. C. 

 Brodie ? It is a weight which in being transported from one 

 combination to another is not "distributed." What does a 

 compound weight represent to him ? It is a weight which in 

 the course of chemical operations is divided, is " distributed.-" 

 Finally, what does he term the " unit of ponderable matter " ? 

 The weight of one volume, say of 1000 cub. centims., of a gas 

 or of a given vapour. 



Let us suppose that Sir B. C. Brodie had accepted his hy- 

 pothesis a 2 instead of stopping at the hypothesis «. All his 

 units would then become equal to our molecules ; those of his 

 " units " which were not " distributed " would have been 

 identical with those of our molecules (mercury, for instance) 

 which we consider as composed of a single atom. Those of 

 his "units" which were "distributed" would have been 



