84 



JOURNAL OF THE 



for instance, copper or mercury or iron be looked upon 

 as a spjcies of isomerism. Such speculations are of lit- 

 tle use, however, and quite apart from our present pur- 

 pose. 



I have found this table very useful in teaching ele- 

 mentary chemistry and it can most profitably be made 

 the basis of the entire course. Thus in the first four 

 groups the left triad contains the elements most closely 

 resembling- the T3^pes. In the last three they are to 

 be found in the rig-ht triads. As to natural occurrence 

 of the elements, in the first four groups those in the 

 left triads occur in the same compounds and generally 

 in connection with the type ; those in the right triads 

 occur as the type or as sulphides or are free. In the last 

 three groups this is reversed. The right triad ele- 

 ments occur as the types and the left triad as the type 

 or as oxides. So too the properties of the elements 

 show this relation to the types. Take as an example 

 the specific gravities in Group II. 



Be 2.1 



Mg 1.75 



Cal.5 2^n6.9 



Sr2.5 Cd8.6 



Ba3.6 Hgl3.6 



It is not necessary to pursue this part of it at great- 

 er length. The careful teacher will easily work out 



