284 F. W. Clarke—TIodates of Cobalt and Nickel. 
chromate loses six molecules of water readily, holding the 
seventh with much greater firmness. Heated to 130° the salt 
more water, but not the whole, is expelled, the compound at 
the same time undergoing partial decomposition. The den- 
sity of MgCrO,, H,O, was found to be 2°2886 and 22301, at 
Ammonium chromate, (NH,),CrO,, crystallized, 1:9138 and 1°9203, 
12°. Schréder makes it 1-860 to 1°871. 
Ammonium dichromate, (NH,),Cr,O,, 2°1223, 16°, and 2°1805, 17°. 
99 
at 20°, a figure probably much too low. The difficulty in 
dealing with this body arises from the extreme instability of 
Ammonium-magnesium chromate, (NH,),Mg(CrO,),, 6H,O, well 
crystallized, 1°8278 and 1°8595 at 16°. Also 1°8293 at 17°. 
and 2°5966, 19°5°. Schréder er 2°592 to 2°608, a close 
agreement. 
_ These chromate determinations fill up some important gaps 
in this series of salts, and help us to arrive at some interesting 
conclusions. Pettersson has lately shown that selenates have 
molecular volumes exceeding those of the corresponding sul- 
phates by 6 for each molecule of the acid radicle. Thus, the 
molecular volume of sodium selenate is that of the sulphate 
plus 6; that of a selenic alum exceeds that of the correspond- 
ing sulphuric alum by four times 6, and soon. Upon compar- 
ing these and other chromates with Pettersson’s selenates we 
now find that the two series of salts have approximately equal 
molecular volumes; the difference, if any exists, being very 
slightly plus for the selenates. If regularities of this kind can 
be thoroughly established, it will be easy, having the density 
of a chromate, to calculate that of the corresponding sulphate 
or selenate, and vice-versa. This connection between chromates 
and selenates has already been suggested by Schréder, and Miss 
Abbot's figures do much towards confirming it. Miss Abbot 
has also redetermined the density of chromic chloride, CraCls, 
on a beautifully characterized specimen. She finds 2°3572, 
175°, and 238766, 165°. Schafarik’s figure for the substance 
