A HUNDRED YEARS OF CHEMISTRY. 675 



other chemists before him; but the fact was disputed by B^-thollet, 

 and its verity was not established until 1S08. Dalton went a step 

 further, and found that to every element a definite combining num- 

 ber could be assigned, and that when two elements united in more 

 than one proportion even multiples of that number appeared. 

 Thus, taking the hydrogen weight as unity, oxygen always combines 

 with other elements in the proportion of eight parts or some simple 

 multiple thereof, and so on through the entjre list of elementary 

 bodies. Each one has its own combining weight, and this was the 

 law for which Dalton sought an adequate explanation. Fractions 

 of the weights did not appear, fractional atoms could not exist; 

 the two thoughts were connected by Dalton. Chemical union, to 

 his mind, became a juxtaposition of atoms, whose relative weights 

 were indicated by their combining numbers, and so the atomic con- 

 ception for the first time was given quantitative expression. The 

 facts were co-ordinated, the special laws were combined in one 

 general theory, and the mere suppositions of other men were sup- 

 planted by a precise statement, which is a corner stone of chemistry 

 to-day. The doctrine led at once to investigations, it rendered pos- 

 sible the discovery of new truth, chemical formulae and chemical 

 equations were developed from it; without its aid the growth of 

 chemical science would probably have been slow. The nature of 

 the atoms may be in doubt, they may be divisible or indivisible, but 

 the value of the theory is independent of such considerations. It 

 gives adequate expression to known laws, and it can only be set 

 aside, if ever, by absorption into some wider and deeper generali- 

 zation. 



The same year w^hich saw the completion of Dalton's theory 

 (1807) was also signalized by the remarkable discoveries of Sir 

 Humphry Davy, who decomposed the alkalies and proved them 

 to be compounds of metals. In 1810 chlorine, which was pre- 

 viously thought to be a compound, was proved to be elementary, 

 and this fact was emphasized a year later by the discovery of 

 iodine. These researches gave precision to the conception of an 

 element, and prepared the way for later investigations upon many 

 other oxides. All the so-called " earths " — lime, magnesia, alu- 

 mina, and so on — were now seen to be oxy-compounds of metals, 

 and an intelligent interpretation of all forms of inorganic matter 

 became possible. The first step in the chain of research was the 

 discovery of oxygen itself; from that, and from the teachings of 

 Lavoisier, the later discoveries logically followed. 



"While the investigations of Dalton and of Davy were still in- 

 complete, other chemists were actively studying the properties of 

 gases and exploring the fertile border-land between chemistrv and 



