818 Dr. Gladstone on the Relations between the 



Silicon is certainly very similar to the first of this group, tita- 

 nium, but its atomic number 14*2 (reckoning silica to be SiO^) in 

 unconformable. .0 



These metals, whose numbers have a multiple relationship) 

 are not remarkable for being found together in nature. 



The third kind of relationship is where an element, having 

 properties intermediate between those of two other elements, has 

 the intermediate atomic weight. Four instances of this have 

 been noticed*. We observe all four groups in the arrangement 

 of Gmelin given above. They are — 



The metals of the alkalies : — 



Lithium, 6*5. Sodium, 23. Potassium, 39'2. 

 The metals of the alkaline earths : — 



Calcium, 20. Strontium, 43*8. Barium, 68*5. 

 The halogens : — 



Chlorine, 35*5. Bromine, 80. Iodine, 127*1. 

 Sulphur and its congeners : — 



Sulphur, 16. Selenium, 39*5. Tellurium, 64*2. 



The members of the last two groups generally occur together in 

 nature. > 



There are certain analogies which may perhaps lead us t6 

 some understanding of these facts. 



First, in the case where there is the same atomic weight. If 

 the allotropism of an element were carried through all its com- 

 pounds, we should have what occurs in the iron and similar 

 series. The only partial instances of this which I remember are 

 the sulphides of phosphorus, as remarked by Berzelius, and silicic 

 acid, the two conditions of which bear a striking analogy to the 

 two allotropic forms of the elementary silicon itself. 



Secondly, in the case of multiple atomic weights, is there not 

 something analogous to the polymerizing of which we have many 

 examples in organic chemistry ? or to the modifications of a metal, 

 such as mercury, where we have 100 or 200 parts combining 

 with 1 equivalent of chlorine to form a salt ? the diflference 

 in this latter case being, that when we regenerate the metal^ from 

 whatever source, it is always the same mercury. 



Thirdly, in the c^se where an element of intermediate proper- 

 ties has an intermediate atomic weight. I regard this as strictly 

 analogous to the series of homologous bodies so common in 

 organic chemistry. My meaning may be best explained perhaps 

 by a reference to the quasi-metals, or compound hydrogens. We 

 have hydrogen 1, methyle 15, sethyle 29, amyle 71 : the com- 



* See Gmelin's Handbook of Chemistry, part 1 . Dumas also brought 

 forward some speculations on these groups at the Ipswich Meeting of the 

 British Association. 



