28 The Realm of Nature CHAP. 



and Professor Mendelejeff in Russia, and is known as the 

 Periodic Law. This states that if the elements are arranged 

 in the order of the mass of their smallest particles, Le. their 

 atomic weight, they will fall into eight groups of about twelve 

 elements each, and the first, second, third, etc., element of 

 each group bears a strong family resemblance to the first, 

 second, third, etc., of each of the other groups. Some of 

 the groups have many gaps, only seventy elements being as 

 yet known ; but the atomic mass, the density, the melting 

 temperature, the colour and the nature of the compounds it 

 would form with known elements can be calculated and 

 predicted for each of the elements which are absent. Names 

 have even been given to these hypothetical elements, and in 

 at least two cases the elements were subsequently discovered 

 by chemists and found to correspond very closely to the 

 prophetic description. This fact was the strongest con- 

 firmation of the truth of the Periodic Law. If the figures 

 known to chemists as " atomic weights " really correspond 

 to the mass of the atoms of each element, as there is reason 

 to believe that they do, the chief difference between the 

 elements may consist in the fact that their smallest particles 

 contain different amounts of matter ; the extreme cases are 

 uranium and hydrogen, the mass of the atom of the former 

 being 240 times that of the latter. We could imagine a great 

 rock to be quarried into blocks of ninety-six definite sizes, 

 the smallest being only -^Q- of the largest, and ship-loads of 

 these cut and squared stones might be sent to a nation 

 where tools were unknown. These people might use the 

 stones in building houses, but would be unable to change 

 any one size into another until they invented the proper tools. 

 They might be supplied only with sixty or seventy of the 

 sizes, but by studying the weights of these and seeing the 

 order in which they ran they might predict the existence of 

 intermediate sizes. As they could not in the absence of 

 tools change the form or size of the blocks, though recognis- 

 ing their unity of composition, they would look on them as 

 unalterable elements in their building. Similarly modern 

 chemistry has enabled us to understand how it is possible 

 that the elements are merely separate parcels of matter which 



