84 PROCEEDINGS OF THE AMERICAN ACADEMY 



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founded on correct principles. Disregarding the distinction 

 of metals and metalloids, and guided chiefly, though not ex- 

 clusively, by the mode of combination and crystalline form, 

 and bringing together those elements which bear the closest 

 relations to each other, he had arranged the elements in six 

 groups, the properties of each of which are closely related to 

 each other, while they differ widely from those of any other 

 group. The elements of any one group are, for the most 

 part, isomorphous, and from similar compounds. Arranging 

 the elements of any one group according to their relative af- 

 finities, and commencing with the strongest, he had found 

 that the physical properties follow the same progression. As 

 in organic chemistry differences of properties correspond to 

 fixed differences of composition, he had noticed that, in like 

 manner, in these series of inorganic chemistry, similar differ- 

 ences manifest themselves in differences of atomic weights. 

 In the series in which he had classified the elements, the dif- 

 ferences between the atomic weights of the consecutive mem- 

 bers of any one series is always a multiple of some whole 

 number. In one case it is 9, in another 8, in another 6, in 

 another 5, in another 4, and in another 3. He stated that 

 there are some discrepances between the atomic weights, as at 

 present determined, and those required by his theory ; and 

 that, though in most cases they are within the limits of actual 

 error, in others there is a residual. These remarks Professor 

 Cooke illustrated very fully by referring to the group consist- 

 ing of oxygen, nitrogen, phosphorus, arsenic, antimony, and 

 bismuth. He showed that these elements have the same mode 

 of combination ; that they not only unite with the same num- 

 ber of atoms, but that the resulting compounds have similar 

 properties, and form parallel series with the elements. He 

 stated reasons for believing that phosphorus, antimony, and 

 arsenic exist in two allotropic states. He had succeeded in 

 crystallizing arsenic in regular octahedrons which belong to a 

 new allotropic state of arsenic ; which in this state differs in 

 color, weight, and chemical properties from common arsenic. 



