94 Messrs. Johnson and Allen 
towards genera of that family. From all the facts, I am inclined 
to agree with those who think the group should be ranged near 
the Bullide and Cylichnide. 
In regard to the geological range of this family, we have evi- 
dence of its existence as far back as the Carboniferous epoch, 
where it is represented by the genus Acteonina. If we include 
the genus Macrocheilus, however, it would carry the origin of the 
family back to the Devonian. It attained its greatest development 
during the Jurassic and Cretaceous periods,—since which it has 
declined; and, although still represented in our existing seas by 
a comparatively few forms, it may be regarded as a type proba- 
bly destined to pass out of existence during the present geologi- 
cal period. 
Washington, D. C., Dec. 20, 1862. 
Art. XV.—Contributions from the Sheffield Laboratory of Yale 
College.—V. On the Equivalent and Spectrum of Cesium; by 
S. W. JoHNson and O. D. ALLEN. 
IN the last vol. of this Journal a method is described of sepa- 
rating cesium from rubidium by fractional crystallization of the 
bitartrates of these metals.’ The analyses of the bitartrate of 
cesium there given, while perfectly according with each other 
as regards carbon, disagree with the numbers deduced from 
Bunsen’s equivalent to such an extent that we have undertaken 
to ascertain whether the salt was impure or Bunsen’s equivalent 
incorrect. 
From the great care used in preparing the bitartrate and es- 
pecially from the fact that its spectrum remained unaltered 
though the salt was repeatedly recrystallized, we were inclined 
to suppose that Bunsen had not operated with a pure substance. 
This might easily happen on account of the small quantity of 
material at his disposal without at all detracting from the merit 
of this distinguished chemist. ; 
uantity of bitartrate of cesium purified by concentrating 
. m 
tion of a little hydrochloric acid. : 
We thus obtained an amorphous mass of a pure white color 
? Observations on Cesium and Rubidium, by O. D, Allen, vol. xxxiv, pp. 367-875: 
