134 The Evolution of Chemistry. 



disputed points between various ratios as to which is the 

 true atomic weight, and in determining atomic weights not 

 otherwise ascertainable. In 1821 Mitscherlich pointed out a 

 law of crystallography that has been used as a third method 

 of determining atomic weights. It has been found that, as 

 a rule, the similar combination of atoms without regard to 

 their chemical natures gives crystals of similar forms. When 

 crystals isomeric in form but different in composition occur, 

 they are pretty certain to be built up of molecules in which 

 the atoms are grouped alike. Knowing this fact, we can 

 know the number of atoms in an unknown group by com- 

 paring it with a known, and if all the atoms in the group 

 but one have had their weights determined, that one is 

 easily calculated. Therefore we have in this method a 

 means of confirming results gained by other methods. A 

 fourth method has lately been devised, and is known as the 

 periodic law. It was first presented a few years ago by a 

 Kussian chemist called Mendelejeff. According to this law, 

 all the leading properties of an element are functions of its 

 atomic weight. Given the atomic weight of any element 

 and its place upon a spirally ascending expanding curve, 

 distance being arranged proportional to weight, and its 

 chief properties may at once be predicted. Of course it 

 follows that the reverse is true. Given the properties of the 

 element, and the atomic weight can be approximated from 

 the place where it belongs on the spiral. If the whole 

 series is bisected from top to bottom, paramagnetic elements 

 will all be found in one half and diamagnetic in the other. 

 All related groups like chlorine, bromine, and iodine will be 

 found almost directly one above the other. In this way 

 every element takes a place by its weight that answers to its 

 leading properties. On such a spiral a number of gaps are 

 found where undiscovered elements are believed to belong. 

 Soon after enunciating the law, Mendelejeff called attention 

 to two of the lower gaps then existing, and described the 

 properties of the elements that should belong there. He 

 called the hypothetic elements eka-aluminium and eka- 

 boron. Since then two elements possessing properties 

 similar to those described by Mendelejeff have been found 

 and named, by their respective discoverers, gallium and 

 scandium. 



In atomic weights, ease of reduction, melting points, 

 specific gravities, power of oxidation, ability to decompose 

 water, methods of being attacked by acids or alkalies, 



