207 
with the atomic weight = 96, and forming a triad with indium 
and thallium. If, however, the experimental determination of 
the atomicity of gallium pass through the same stages as 
the atomicities of indium, yttrium, and other members of 
the series, its atomic weight will be represented by the sub- 
multiple and proportional numbers 48 and 72.* 
Just as silver and copper are analogues of each other, and 
are frequently associated in nature; and, just as their 
homologues, cadmium and zinc are analogues, and are also 
found together, so is yttrium the analogue of x = 69, and 
will be found associated with it in nature. Now, if x =69 
be not the terbium of Mosander and Delafontaine, and the 
researches of Bahr and Bunsen render the existence of this 
element doubtful, it is probable that a? =69 is cerium, as 
this element and yttrium are nearly always found asso- 
ciated in the mineral species cerite and yttrocerite. More- 
over, it will be observed that cc = 69 is just 15, or 0'75 the 
atomic weight of cerium, according as it is regarded as 46 
or 92. Mendeleeff and other chemists have already proposed 
138 as the atomic weight of cerium, *f* which is double that 
of ^ = 69. MM. Hildebrand and Norton have recently 
obtained cerium, lanthanum, and didymium in a massive 
state, and have thereby been able to investigate some of the 
physical properties of these rare metals. J According to 
these experimenters the specific gravities of Ce, La, and Di, 
range between 6 and 6*7. Bearing in mind that elements 
of approximately the same atomic weights and specific 
gravities generally belong to different series, and that the 
* From a calcination of the gallo-ammoniacal alum, M. Lecoq de 
Boisbaudran has recently found for gallium the equivalent 70 03, and 
from a calcination of the nitrate, 69 ’6 — Comptes Bendus , April 15th, 1878. 
The researches of M. Berthelot on the specific heat of gallium indicate, 
however, a higher equivalent for the metal than 70’03, as the atomic heat 
calculated from this determination (5 52 solid) is lower than that of any 
other metal except silicium. — Ibid, April 15th, 1878. 
f Ann. Chem. Pharm. Suppl. viii., 185 — 190. 
X Chem. Soc. Journal, 1876, vol. ii., 276. 
