36 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



be continuously subdivided until upon further subdivision 

 it ceases to exist in the form known to us as iron, at that 

 point we may be said to have reached an * atom/ one of the 

 ultimate components of matter. 



Recent physical researches suggest that the atom itself 

 can be further subdivided into still smaller particles known 

 as electrons, but setting aside this possibility, for the purposes 

 of the chemist it suffices to define the atom as the smallest 

 existing particle of an element. 



This idea of the atomic structure of matter is a very old one 

 and was held by the ancients, and entered largely into the 

 conceptions of Robert Boyle and other chemical philosophers. 



It is to the genius of Dalton that we owe a development 

 of the atomic theory, which converted it from a more or less 

 barren speculation into a fundamental and fruitful conception. 

 Dalton was able to show that the atom of any given element 

 was characterised by a definite and unalterable weight which, 

 while too small to be expressed by absolute numbers, could be 

 referred to in terms of the weight of the lightest then known 

 element, viz., hydrogen, which was taken as unity ; thus the 

 atom of iron, e.g., has been found to be 56 times as heavy as 

 the atom of hydrogen. 



Dalton used symbols, somewhat akin to the old alchemical 

 symbols, viz., circles, semicircles and the like, for expressing 

 the ultimate atoms and elements. It was the great Swedish 

 chemist, Berzelius, who introduced the much more convenient 

 method of referring to elements, either by their initial letters, 

 or by the initial letter together with a second significant 

 letter. These are known as the symbols of the elements ; thus 

 the symbol H signifies one part by weight of hydrogen, the 

 symbol sixteen parts by weight of oxygen. 



In order to obtain true values for these relative weights 

 of the elements, which should really express the weights of 

 their atoms as compared with the weight of an atom 

 of hydrogen, it was necessary to extend the conception of 



