REVIEWS 685 



elements. This view is pretty definitely taken in Mr. Jobling's book, and there 

 is considerable evidence in its favour. It seems reasonably well established that 

 metals like platinum and copper owe their catalytic properties to the fact that they 

 can give rise to at least two series of compounds in which the metals possess 

 different valencies. Similarly the activity of the oxides of metals such as thorium 

 titanium, and manganese may be attributed to the polyvalent nature of the 

 metallic element. In the same connection the idea that the instability of the 

 metallic hydride, oxide, or salt is essential, has been borne out in numerous 

 instances, the classical case being that of cupric chloride (in the Deacon process), 

 whose catalytic property had been inferred by Hurter from a comparative study 

 of heats of combination. That the mechanism of catalysis in general is obscure 

 need scarcely be insisted on. Thus in the Claus-Chance process of sulphur re- 

 covery, in which sulphuretted hydrogen is burnt with the required amount of air, 

 it has been found that ferric oxide is an excellent catalyst. During the process 

 the ferric oxide is changed into pyrites, but the pyrites so formed "has some 

 peculiar property attached to it which ordinary pyrites does not possess, for the 

 latter is quite useless as a catalyst." Or again, in surface combustion, it has been 

 found that the activity of the surface is affected by previous contact with the 

 combustible gases. " Thus in most cases exposure to the combustible gas enhances 

 the catalysing factor, whereas contact with oxygen lowers it, though to a smaller 

 degree." An equally mysterious effect is that of " poisoning " the catalyst, the 

 remarkable fact being the disparity between the effect produced and the minute 

 quantity of poison required. Mr. Jobling instances the Haber ammonia process 

 in which iron is the catalyst and sulphur one of its poisons. " The iron used as 

 catalyst becomes quite 'dead' when it contains ro per cent, of sulphur, and is of 

 little use with r J T per cent, impurity even " 



Many other problems are raised and discussed, such, for example, as negative 

 catalysis, the function of promoters, the mechanism of induced reactions, and 

 numerous specific effects observed in particular processes. There is not space 

 to deal with these, but sufficient has been said to indicate the scope of the book. 



There are only one or two slight misprints. On p. 10 Haber's name is twice 

 spelt Heber, and on p. 45 (six lines from top) "pressure" occurs in place of 



" presence." 



W. C. McC. Lewis. 



The Theory of Valency. By J. N. Friend, D.Sc, Ph.D., F.I.C. (Text- 

 books of Physical Chemistry, Edited by Sir William Ramsay, K.C.B., 

 F.R.S.) [Pp. xiv + 192. Second, revised edition.] (London: Longmans, 

 Green & Co., 191 5. Price $s. net.) 



This second edition is, to a large extent, identical with the first. Additiona 

 matter has been introduced into Chapter V, viz. the question of the anomalous 

 position occupied by tellurium in the Periodic System, the valency of the rare- 

 earth elements, and the radio-elements, including a reference to Soddy's concept 

 of isotopes. In treating the inert gases, mention is made of Mathew's attempt to 

 connect valency with cohesion, and the conclusion to be drawn from the solubility 

 of the rare gases in water. Chapter IX on the valency of the elements of Group 1 

 has been largely rewritten. In a later chapter, the account of Werner's theory is 

 extended by a brief discussion of auxiliary valencies. In the most important 

 chapter of the book, namely, that dealing with electro-chemical theories of valency, 

 an attempt is made to bring the account up to date by a short review of modern 

 views on atomic structure and the electronic theory of valency. One or two 



