ULTIMATE CONSTITUTION OF MATTER 85 



equals 8. To take illustrations from each of the groups 

 IV., V., VI., VII., we find that carbon forms CH 4 and CO 2 , 

 nitrogen NHa and N2O5, sulphur SOs and SH2, and iodine 

 I 2 O 7 and IH. 



As yet the expression of the Periodic Law is tentative 

 and provisional. The fact that it explains many hitherto 

 inexplicable phenomena indicates that, when it is completely 

 and justly formulated, it will account for many more. It 

 exemplifies the grand function of Science, to marshal the 

 apparently unrelated and uncoordinated facts of the uni- 

 verse. Boyle's rabble of elements is already a disciplined 

 army. The discovery of the ' ' periodicity ' ' of their properties 

 has given the chemist an entirely new grasp of the elements. 

 As a mere memoria technica, Mendeleef s table is of immense 

 value to the student. Any element of which he thinks is 

 no longer an isolated unit, with properties peculiar to itself. 

 Its place in the table shows him what its characters must be, 

 relatively, both to the horizontal series and to the vertical 

 group to which it belongs. But the Periodic Law, although 

 we do not' yet know its full meaning, is far more than an 

 aid to the memory. It is prophetic as well as retrospective 

 or explanatory. It has called attention to many of the 

 shortcomings of chemical science, and foretold how they 

 may be corrected. The incorrect atomic weights of elements 

 which would not fit into the scheme have been set right. 

 The existence of vacant spaces in the table has led to the 

 search for new elements ; it has indicated their alliances, 

 pointed to the minerals in which they might be looked for, 

 and thus led to their discovery. Compounds which were 

 supposed not to exist have been found after the position of 

 an element in a group which usually formed such compounds 

 had been admitted. 



If Mendeleef's theory is correct, it follows that the 

 number of elements is strictly limited. Some of them 

 have not yet been discovered, but Mendeleef's prophecy 

 that the vacant spaces will be gradually filled has already 



