396 
DR. A. E. H. TUTTON ON THE 
It has been shown in the separate memoir on the chemistry of these double 
selenates that a successful and ready method of preparing ferrous selenate in solution, 
which automatically preserves it in the ferrous condition by retarding oxidation by 
atmospheric oxygen, is afforded by acting on pure ferrous sulphide in the cold with 
moderately dilute selenic acid. The especial difficulty of obtaining potassium ferrous 
selenate from the filtered solution of ferrous selenate, after admixture with the equal 
molecular proportion of potassium selenate in fairly concentrated solution, has been 
overcome by making the preparations in the coldest winter weather, the crystals 
successfully obtained having all been deposited during the four coldest nights of early 
January, 1918, which were distinguished on Dartmoor for their exceptionally low 
temperature. The crystallisation occurred in a partial vacuum over oil of vitriol, 
under the receiver of an air pump. The goniometry and optics were carried through 
on the succeeding day, immediately after removal from the mother liquor, in each case, 
during the four or five hours of the transparent life of the crystals and in a very cold 
room without fire or other artificial heating. 
Potassium Ferrous Selenate K 2 Fe(Se0 4 ) 2 .6H 2 0. 
The four crops of beautifully clear, transparent, pale green crystals obtained of this 
remarkable salt were produced during four very cold, nights in the manner just 
described, and given in more detail in the 1 Proceedings ’ memoir on the chemistry of 
the group (p. 360). They were derived from three separate preparations, only one of 
which yielded a second crop on the next, also very cold, night. The temperature of 
the laboratory on all four nights was only just a fraction of a degree above 0° C., there 
being from 8° C. to 10° C. of frost outside. Further crops from these preparations, 
after filtering from oxidised ferric products, and all those derived from the numerous 
other preparations made in cold, but not such extremely cold, weather, consisted only 
of very minute crystals, quite unsuitable for goniometrical examination, which 
corresponded with those which Tops0e states are produced at ordinary temperatures 
on moderately mild or warm days, and which he describes as having the composition 
K 2 Fe (Se0 4 ) 2 . 2H 2 0 and as being of probably triclinic symmetry. 
According to Tops0E the decomposition of the crystals of the hexahydrated salt is 
due to their conversion into this dihydrated salt at a temperature but a few degrees 
above the freezing point of water. Confirmatory evidence of this will be afforded 
when the density is discussed, as the author found that the density rises continuously 
and quite markedly during every half-hour of the life of the crystals after decomposi¬ 
tion once starts. 
There is no doubt that this potassium salt of the iron group is unstable for the 
reason that in the regular descent in stability, which the author has shown to be 
general for all the groups of salts of this hexahydrated series (including the double 
sulphates), from the caesium salt through the rubidium salt to the potassium salt, the 
