68G Dr. A. E. H. Tutton [March 14, 



The valency theory of Barlow and Pope may or may not in the 

 seqnel prove to be correct, and some facts have recently been brought 

 forward by Barker which tend to show that it will not hold in many 

 cases of inorganic substances. Barker, who has had the good fortune 

 to have worked in St. Petersburg with von Fedorow for more than a 

 year, shows that, as the lecturer has always held, the true unit of 

 volume is the molecular or atomic volume, as determined for the par- 

 ticular substance itself. The molecular volume is determinable by 

 dividing the molecular weight of the substance by the specific gravity 

 of its crystals at a definite comparable temperature, such as 20° C, 

 but the determination of the atomic volume offers peculiar difficulty, 

 and so far only comparative and indirect methods have been em- 

 ployed, chiefly by Sollas. By taking the volumes of the spherical 

 units to be proportional to the atomic volumes (not those of the 



Fig. 6.— Fedorow's Stereohedra. 



element in the free state, as enormous compression occurs on com- 

 bination), and also determining the amount of free interstitial space 

 by comparative methods of calculation, Sollas has achieved some 

 remarkable explanations of the crystallographic characters of the two 

 polymorphous forms of silver iodide and of the three forms of 

 titanium dioxide, rutile, anatase, and brookite. We have as yet no 

 guidance from von Fedorow as to the nature of the atomic units and 

 the volumes which they occupy. It would not be surprising if the 

 valency volumes of Barlow and Pope, in the cases of those elements 

 for which their theory appears to work in a satisfactory manner, turn 

 out to be identical with the atomic volumes as determined by the 

 method of Sollas. As regards the compounds of carbon and hydrogen, 

 Barlow and Pope have been most successful in accounting for crystallo- 

 graphic and chemical relationships, and it is at least significant that 

 both Tie Bas, from experimental work on the molecular volumes of 

 liquid hydrocarbons, and Traube from an entirely different point of 

 view, coincide in assigning the relative volumes 4 and 1 to carbon 

 and hydrogen atoms in combination respectively. If Traube's results 



