THE REVIVAL OF INORGANIC CHEMISTRY. 303 



maybe wholly or in part replaced by pyridine, water, acid radicals, and 

 other groups. These groups are supposed to be arranged symmetric- 

 ally about the metallic atom, forming a radical, which, according to 

 its nature, can combine as a whole with metals, halogens, or other 

 positive or negative groups. Thus, in the compound 0O(NH 3 )6Cl 3 , 

 cobalt forms with NH :j a radical, which combines as a whole with the 

 three chlorine atoms; in (PtCl 6 )K 2 the two potassium atoms are com- 

 bined with the whole, group PtCl 6 and not attached to any one part of 

 it; the same applies to (NH 4 )01, and to K 2 (S0 4 ) and K 4 (FeCN) 6 . In 

 the formation of these radicals the bivalent NH :! , the neutral H 2 0, and 

 the univaleut Clean replace each other indiscriminately; the valence 

 theory is, therefore, practically thrown overboard entirely and in place 

 of combination by bonds we have an extension of the old theory of 

 molecular compounds applicable alike to the metal-ammonias, t lie 

 ordinary oxygen salts, the double halides, and the compounds with 

 water of crystallization. It is yet too soon to predict the future of this 

 hypothesis, which has already won numerous active adherents. It is 

 scarcely too much to hope that it will lead, perhaps with some modifica- 

 tions and extensions, to a more comprehensive theory of structure, and 

 to a clearer definition of the as yet only vague conception of valency. 

 It is the broadest generalization of inorganic chemistry since the dis- 

 covery of the Periodic Law, and shows that inorganic chemists are no 

 longer willing to be mere imitators and to close their eyes to the exist- 

 ence of whole groups of bodies which do not tally with current theories, 

 and are beginning to see that in these is to be sought the key to a 

 broader inorganic chemistry. 



The slow development of inorganic chemistry during the period from 

 1830 to 1865, as compared with that of organic chemistry, was due, as 

 has been seen, in part to the greater breadth and greater diversity of the 

 field, to the relative absence of leading ideas and leading motives, and to 

 the comparative tractability of carbon compounds as compared with 

 inorganic compounds under the restrictions of the experimental methods 

 in vogue. Prout's hypothesis and allied speculations gave a working 

 hypothesis for a limited number of investigators, but the uncertainty 

 of the atomic weights, which in part was conditioned by the imperfec- 

 tion of analytical methods, prevented any satisfactory results being 

 reached. Absolute purity of materials and absolute accuracy of analyt- 

 ical methods are not of the first importance to the organic chemist, to 

 whom errors of one or two points in the first decimal are seldom of any 

 significance. To the atomic-weight chemist, on the contrary, accuracy 

 is the very first point to be considered; not only must his material be 

 absolutely free from impurities, but his methods must be beyond 

 criticism, and it is only with the increasing perfection of analytical 

 methods, admitting not only of quantitative determinations of the 

 greate~st accuracy, but also of the detection of traces of impurities 

 which for ordinary purposes are negligible, that this kind of work has 



