246 THE NUMERICAL RELATION 



Sulphur, Selenium, and Tellurium, the other of Molybdenum, Vanadium, Tungsten, 

 and Tantalum. The atomic weights of the first are all equal to 8 + w 8 ; those of the 

 second to 4 -|- n 8. The sub-series exhibit marked analogies, as well as certain difi"er- 

 ences. They resemble each other chiefly in that the members of both form analogous 

 acids with Oxygen, while they differ in that, though the members of the first sub-series 

 foi-m compounds with Hydrogen, those of the second do not. The isomorphism of the 

 members of each sub-series among themselves, with the exception of Vanadium, is com- 

 plete ; but there seems to be no proof of any isomorphism between the sub-series. 

 Johnston attempted to establish the isomorphism of Chromic and Molybdic Acids from 

 the red variety of Molybdate of Lead from Rezbanya, which he supposed to be a Chro- 

 mate ; but the fact has been disproved by G. Rose, who has shown that the supposed 

 Chromate is a Molybdate mixed with a small amount only of Chromate. There seems, 

 nevertheless, to be some reason for believing that Chromic Acid may replace Molybdic 

 Acid to a certain extent. If this is proved, it establishes another link of connection 

 between the members of the two sub-series, since Chromic Acid is isomorphous with 

 Sulphuric Acid. For the present, however, we must regard them as sub-series, related, 

 but distinct, the second being in a measure supplementary to the first. They are dis- 

 tinguished in the table by printing the names of the second sub-series a little to the 

 right of those of the first, and the fact that their atomic weights are intermediate to 

 those of the first, I have indicated to the eye by giving to the names also an interme- 

 diate position. 



The analogies between Oxygen and Sulphur are so numerous, that, were we to place 

 Oxygen in but one series, we should place it in this. HO and HS, HO2 and HS2, resemble 

 each other very closely, as do also the Oxygen salts the corresponding Sulphur salts. 

 Moreover, there can be no doubt in regard to the isomorphism of the two elements, 

 since it has been established upon the authority both of Mitscherlich and Becquerel, 

 and from two different compounds. The only doubtful case in the series was that of 

 Vanadium, which in some of its properties resembles Arsenic more closely than it does 

 Molybdenum. The reasons for giving it the place which it occupies were the facts that 

 its acids correspond to those of INIolybdenum, and that it forms remarkably highly 

 colored oxides which are repeated also in INIolybdenum. It is true that the properties 

 of the element itself are not those we should expect from the position which it occupies 

 in our table ; yet, if it were placed in the Six Series, it would fall between Phosphorus 

 and Arsenic, which on the whole it resembles less than it does Molybdenum, for al- 

 though it is combustible, yet neither it nor its oxides are volatile. I consider it, there- 

 fore, as a member of the Eight Series, but affiliating very closely with the Six. Its 



