president's address. 9 



constructed a reversible chemical equation: Metal — phlogiston = Calx ; 

 Calx + phlogiston = Metal. 



It is difficult to say when the first element was known to be an 

 element. After Lavoisier's overthrow of the phlogistic hypothesis, the 

 part played by oxygen, then recently discovered by Priestley and 

 Scheele, came prominently forward. Loss of phlogiston was identified 

 with oxidation; gain of phlogiston, with loss of oxygen. The scheme 

 of nomenclature (Methode de Nomenclature chimique ') published by 

 Lavoisier in conjunction with Guyton de Morveau, Berthollet, and 

 Fourcroy, created a system of chemistry out of a wilderness of isolated 

 facts and descriptions. Shortly after, in 1789, Lavoisier published his 

 ' Traite de'Chimie,' and in the preface the words occur: ' If we mean 

 by " elements" the simple and indivisible molecules of which bodies 

 consist, it is probable that we do not know them; if, on the other 

 hand, we mean the last term in analysis, then every substance which 

 we have not been able to decompose is for us an element ; not that 

 we can be certain that bodies which we regard as simple are not them- 

 selves composed of two or even a larger number of elements, but 

 because these elements can never be separated, or rather, because we 

 have no means of separating them, they act, so far as we can judge, as 

 elements ; and we cannot call them ' ' simple ' ' until experiment and 

 observation shall have furnished a proof that they are so.' 



The close connection between ' crocus of Mars ' and metallic iron, 

 the former named by Lavoisier ' oxyde de fer, ' and similar relations 

 between metals and their oxides, made it likely that bodies which 

 reacted as oxides in dissolving in acids and forming salts must also 

 possess a metallic substratum. In October 1807 Sir Humphry Davy 

 proved the correctness of this view for soda and potash by his famous 

 experiment of splitting these bodies by a powerful electric current into 

 oxygen and hydrogen on the one hand, and the metals sodium and 

 potassium on the other. Calcium, barium, strontium, and magnesium 

 were added to the list as constituents of the oxides, lime, barytes, 

 strontia, and magnesia. Some years later Scheele's ' dephlogisticated 

 marine acid,' obtained by heating pyrolusite with ' spirit of salt,' was 

 identified by Davy as in all likelihood elementary. His words are : 

 ' All the conclusions which I have ventured to make respecting the 

 undecompounded nature of oxymuriatic gas are, I conceive, entirely 

 confirmed by these new facts. ' ' It has been judged most proper to 

 suggest a name founded upon one of its obvious and characteristic 

 properties, its colour, and to call it chlorine.' The subsequent dis- 

 covery of iodine by Courtois in 1812, and of bromine by Balard in 

 1826, led to the inevitable conclusion that fluorine, if isolated, should 

 resemble the other halogens in properties, and much later, in the able 

 hands of Moissan, this was shown to be true. 



