SIE B. C. BEODIE ON THE CALCULUS OF CHEMICAL OPERATIONS. 785 



the world of chemistry on which the attention may advantageously be concentrated. 

 Ordinary language is too vague and too diffuse for the purposes of science, and in 

 chemistry especially the facts are so numerous and so complicated that it is only when 

 embodied in a concrete form that they can be stored in the memory, and become the 

 object of reflection. Hence even an imperfect and material picture may, in a certain 

 epoch of the development of the science, be found of indispensable utility. 



The actual theory of chemistry may be regarded as an expansion of the hypothesis of 

 Dalton\ In science, as in other spheres of thought, hypotheses often pass without 

 question which come to us recommended by early use, and by even a short tradition ; 

 and when embodied in symbolic language, and thus intimately blended with our con- 

 ceptions, they are readily mistaken for facts. No statement, perhaps, would receive 

 more universal or unqualified assent from chemists than that the molecule of ammonia 

 contains three atoms of hydrogen, that ethylamine is derived from ammonia by the 

 substitution of an atom of ethyl for an atom of hydrogen, and that the reason why 

 there are three, and only three, such derivatives of ammonia is, that each derivative is 

 formed by a repetition of the same process, and that, the molecule of ammonia contain- 

 ing only three atoms of hydrogen, this process can only be repeated three times. These 

 conclusions are regarded as so certain as to be almost removed from discussion. But never- 

 theless they cannot but partake of the hypothetical nature of the theory in Avhich they 

 originated. It is only because our primary hypothesis has led us to take this peculiar 

 view of the atomic constitution of ammonia, and to express it by the symbol NH3, that 

 chemists have adopted these further hypotheses as to the nature of the process by which 

 ethylamine is formed, and as to the cause of the limitation which exists in regard to the 

 number of these derivatives. Again, the theory of atomicity has a similar origin. Glycol 

 is supposed to be derived from two molecules of water by the substitution of an atom of 

 the diatomic hydrocarbon ethylene for two atoms of hydrogen; the diatomic radical, 

 in the forcible language of the distinguished discoverer of this £ubctance, "welding" and 

 " riveting" together the residues of the two molecules of water*. What is this 

 doctrine 1 It is simply the expression in language of the relation of the symbols 



> -o 



; C, Hi 



H 



And if the course of the science had been, as might have been the case, such as to have led 

 us to a different view of the atomic constitution of these substances, we should have a dif- 

 ferent order of chemical ideas, and the theoiy, in its actual form, would never have existed. 



* " Toutes CCS mol(5cules sont cimentees, en quclqiie sorte, par dcs dlcments polyatomiqucs, qui posevdent la 

 propridte de se soudcr les una aux autres." 



" II est bien entendu que dans I'cthyle lui-mume Ics atomcs sont rivi^s ensemble par le carbono tetratomiqne." 

 — WcETz, ' Lemons de Philosopbie Chimique,' 1864, pp. 138-140. 



