LECTURE VIII.] HISTORY OF CHEMISTRY. 139 



the establishing of the chemical molecule, and to the theory 

 of types. Simultaneously, the conception of the equivalent 

 assumes a more fixed form, and is distinguished from that of 

 the atom ; it is recognised that the atoms are not equivalent, 

 but are of different values in combination ; the theory of 

 atomicity is developed and this stimulates the determination 

 of rational constitution as we now understand it. 



Let us next make something more than a mere bird's eye in- 

 spection of this period, rich as it is in discoveries and hypotheses. 

 We shall now subject it to a minute examination. In doing so, 

 we find that the development of chemistry during the last sixty 

 years is not inferior in interesting and important episodes to 

 that of any period in the science. The participation in this 

 development is a constantly increasing one, and it is a difficult 

 task to seek out from the enormous mass of material which was 

 elaborated during this period, the things that were important 

 and conducive to progress, to state the development of the ideas 

 in such a way that they shall be at once logical and in accord- 

 ance with the actual facts, to do justice to every one, and yet 

 not to lose the thread over details or questions of priority. 



The history of this epoch has never yet been described in 

 a connected manner. 28 In venturing upon the attempt to do 

 this, I am well aware that an objective representation of the 

 period is scarcely possible, and that I play the part rather of 

 critic than of historian. Still I have endeavoured to make my 

 exposition of some value from the fact that I have always been 

 careful to arrive as nearly as I could at the truth, and not to 

 permit myself to be led by prejudices or personal matters. 



The conception of equivalence might have led to that of 

 replacement or substitution, since the quantities of two acids 

 were equivalent when they saturated the same quantity of a 

 base. The acid in a neutral salt could thus be replaced by its 

 equivalent, without the neutrality being interfered with. The 

 word " replacement " received further justification after Mitscher- 



28 Wurtz's Histoire des doctrines chimiques only appeared during the 

 printing of the first edition of these lectures, and Kopp's Entwickelung der 

 Chemie in der neueren Zeit appeared some years afterwards. 



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