12 
NOTES ON IODIDE OF POTASSIUM, ETC. 
or crystals, and does not entirely separate when the salt is left to drain for a consi¬ 
derable length of time. A temperature of 212° also fails to abstract this moisture. 
On a large scale the crystals are submitted to a temperature of upwards of 300° 
for many hours. After this application, if they are reduced to powder and sub¬ 
mitted to a still higher temperature, they do not lose weight, but if the heat be 
gradually increased to the fusing-point, a loss is at once perceptible, and the 
salt indicates the presence of iodate of potash. If these crystals are kept for a 
length of time or exposed to the air, the deliquescent alkali contained in the in¬ 
terstices will attract sufficient moisture to account for the percentage estimated 
in Mr. Clayton’s analysis in most instances. When much care is bestowed upon 
the preparation of this salt the crystals possess great beauty, occurring in opaque 
or ivory-white cubes or blocks, sometimes in flat plates or slabs, weighing from 
a few grains to several ounces each. The opacity, transparency, and size of the 
crystal are entirely in the hands of the manufacturer; but uucler any circum¬ 
stances the crystals are never perfect, one or more of the faces or sides of the 
cube being always imperfect. After operating upon large quantities with un¬ 
remitting care and adopting every contrivance, I never succeeded in producing a 
perfect cube. If the imperfect side of one of these cubes be placed upwards in 
a glass tube, and heat applied, the form of the crystal can be preserved and the 
moisture, if required, estimated (this side being composed of the edges of the pla tes 
between which the steam can escape) ; but if, on the other hand, the imperfect face 
of the cube be placed downwards and weighted on a platina or porcelain dish, on 
the very careful application of heat the cube will be shattered or cleaved into 
thin plates or small cubes, it is therefore evident if these crystals were not 
porous or one or more of the faces imperfect, the salt could not be rendered 
anhydrous without the crystals or cubes being shattered to pieces. The fine¬ 
ness and beauty of the shell that surrounds the sides of the cube will not 
allow the steam formed to escape on the application of heat. If one of the 
plates separated by heat or cleavage be examined under the microscope, a saline 
body will be observed to be dried on the ivory surface. If this surface be 
moistened with a slight breath of steam, upon the application of turmeric paper, 
a brown impression will be left upon the paper, and the whole of the alkali may 
be thus removed from both surfaces, and the thin plate or beeswing left will be 
found to be neutral, although if the crystals be dissolved in water they will 
scarcely give an alkaline reaction. 
'The entire absence of bromine in all the samples of iodide examined by Mr. 
Clayton seems remarkable, as we find bromine associated with iodine in much of 
the fucus and ulva used in the manufacture of kelp ; and as British manufac¬ 
turers do not collect or separate the bromine, the question arises, what becomes 
of it? 
I had occasion to remark some time ago, that when testing bromide of potas¬ 
sium for the detection of iodide, starch failed readily to indicate iodine in an 
alkaline solution. This fact has been long since known with regard to ozone. 
To be perfectly sensitive to the influence of ozone the cubic iodide requires to be 
exactly neutralized with hydriodic acid.* From a neutral solution the salt then 
crystallizes in nearly transparent or pale-yellow octohedrons, with rhombic 
bases, or similar in geometrical form to iodine when crystallized from an alco¬ 
holic solution or separated by sublimation. I drew attention to the value of 
this neutral iodide some years ago in photography, and it has since become a com¬ 
mercial article amongst photographers. With regard to the iodate often con¬ 
tained in some of the French or foreign iodide, I believe it to be partly due to 
the processes they adopt differing from those of the manufacturers of this country. 
* The foreign iodide does not answer for this purpose, as it often contains iodate of 
potash. 
