Influence of Atomic Weight. 



321 



in the Table of atomic weights (p. 312) corresponding to a 

 monad with atomic weight approximating to 37'6 ; for they 

 are already occupied by K and CI. For similar reasons it 

 cannot be a dyad ; it must therefore be either a triad, and 

 belong to group III., or a tetrad and belong to group IV. 

 But in the latter case its atomic weight ought, according to 

 the mean of the atomic weights of the atom-analogues of a 

 tetrad position, to be either 140 or 164, and not 150*4 ; whilst 

 its properties and those of its compounds ought to resemble 

 either those of Sn and Pb or those of Zr and Th ; but this is 

 not the case. Consequently there only remains the triad po- 

 sition in group III. and atomic weight 113. 



(3) To the Determination of the Properties of still undisco- 

 vered Elements. — The Table of elements (p. 312) shows that there 

 are still many vacant places to be filled by elements yet to be 

 discovered. But from what has been already said, it is easily 

 seen that the periodic law renders it possible to predict, within 

 certain limits, the properties of still undiscovered elements 

 when those of their atom-analogues are known. 



At the time when Mendeljeff published his memoir, the 

 position between Al and In was still unoccupied ; and from 

 the properties of the atom-analogues of that position, viz. Al, 

 In, and Zn, he made predictions as to the properties of the 

 missing element. This then unknown element he termed 

 eka-aluminum. Since that time gallium has been discovered, 

 and its properties described by Lecocq de Boisbaudran, from 

 which properties it appears that it is the missing element eka- 

 aluminium. In order to show, therefore, the extent to which 

 the predictions of Mendeljeff have been verified, the predicted 

 and the actual properties of gallium are placed side by side in 

 the following Table*: — 



Predicted Properties. 



(1) Atomic weight = (>8. 



(2) Specific gravity = 59. 



(3) Easily fusible. 



(4) Its chloride will be volatile. 



the 



(5) Will be easily obtained in 

 metallic state. 



(6) Its sulphide will be insoluble in 

 water. 



(7) Will be discovered by the spectro- 

 scope. 



(8) Precipitated by BaC0 3 in the cold. 



(9) Oxide soluble in ammonia. 

 (10) Will form an alum. 



Actual Properties. 



(1) Atomic weight =G9-8. 



(2) Specific gravity = 5*956. 



(3) Melts at 30° C. 



(4) G-allium chloride is easily fusible 

 (melting-point 73° C.) and volatile. 



(5) Can be easily obtained in the me- 

 tallic state. 



(6) Gallium sulphide is insoluble in 

 water. 



(7) Discovered by spectroscope, giving 

 characteristic violet bands. 



(8) Precipitated by BaCOg in the cold. 



(9) Oxide partially soluble in ammonia. 

 (10) Forms an alum. 



(4) To the Correction of those Atomic Weights which are 

 * See also a paper by Muir (Phil. Mag. April 1877). 



