56 REPORT—1857. 
and the vapour arises, and then add a few drops of liquor ammoniz, which will be im- 
mediately charged with a large excess of iodine in solution. This may be poured into 
a bottle, and more iodine and ammonia added until the requisite supply is obtained. 
Ammonio-iodide of Gold.—Gold-leaf when placed in the iodine solution instantly 
turns black (or purple if the solution be diluted), and immediately dissolves, like sugar 
in water. If left to evaporate spontaneously in some quantity, we obtain black four- 
sided prisms of the ammonio-periodide, which readily dissolve in water ; and if a very 
weak solution be exposed for some months to the direct action of the sun’s rays, a slight 
precipitate appears, and a drop or two of the clear solution furnishes a most striking 
microscopic object, both as to crystalline arrangement and richness of tint, when placed 
in polarized light. 
The application of gentle heat to the ammonio-periodide expels iodine, and leaves 
white crystals of the ammonio-iodide. 
The addition of ammonia to the ammonio-iodides is generally attended with inter- 
esting results. In the present case, by adding ammonia to the periodide of gold we 
obtain a crop of minute six-sided crystals terminated by hexahedral summits, and acted 
on by polarized light. 
If a drop or two of the iodine solution be spread on a slip of glass, and two or three 
square inches of gold-leaf be dissolved in it, a very slight application of heat will quicken 
the crystallization and cause it to assume the arborescent form, or if the little forest 
be not quite satisfactory, a second solution, by means of a drop of water and re-cry- 
stallization, will give what is required. ‘The slip of glass must now be held over the 
spirit-lamp to drive off the iodine, at first slowly, and continued there till the evapo- 
rable elements are gone and nothing remains but metallic arborescent gold as a charm- 
ing microscopic object. It may be protected by Canada balsam, as in the example 
exhibited. The ammonio-periodide of gold is a very valuable ingredient in the toning 
bath for photographic pictures. Before adding it to the solution of hyposulphite of 
soda, or using it as an independent bath, it should be dissolved in water and boiled in 
a test tube, or an evaporating dish, to drive off any free iodine, and then the quantity 
added to the bath depends upon the tint required for the picture, which may be brought 
up to a deep purple. 
The solution of iodine in ammonia may also be successfully used in separating the 
pure metal from gold ore obtained at “the diggings,” where the per-centage of gold 
is verysmall. In a commercial point of view this solution might be in some cases even 
more available than mercury, and the iodine could be easily collected and used for 
further experiments. 
Ammonio-iodide of Silver—Gmelin says of the ammonio-iodide of silver, that 
‘‘unfused iodide of silver absorbs with evolution of heat 3°6 per cent. of ammonia, 
and forms a white compound, which on exposure to the air gives off ammonia and 
turns yellow again.” 
The phenomena are far more interesting when silver-leaf is added to the ammonia 
solution of iodine. The metallic silver is immediately dissolved, and when a few 
drops are placed on a slip of glass beautiful brushes of prismatic crystals shoot out in 
all directions, which may be mounted as a microscopic object in Canada balsam after 
the excess of iodine is spontaneously evaporated. Under polarized light the colours 
of the crystals are brilliant in the extreme, 
A few drops of rather strong ammonia added to these crystals of ammonio-iodide of 
silver on the stage of the microscope, produce a crop of hexagonal and triangular 
plates, which attain soon after their formation a variety of tints rivalling the reflexions 
from the facets of the diamond, They must of course be viewed as opake objects, 
but unfortunately they are not permanent and cannot be preserved in the cabinet. 
This is also the case with similar crystals of iron similarly formed. 
Ammonio-iodide of Mercury.—The phenomena in forming this compound are varied — 
and interesting. Mercury is added to the iodine solution, and after the application — 
of heat and the addition of a little water, a few drops on a slip of glass give bundles 
of prismatic crystals similar to those of silver, and acted on with the same energy by 
polarized light. If ammonia be added to these crystals they are immediately covered 
with tufts of snowy whiteness, and by degrees these are converted into ruby-coloured — 
hexagonal prisms which are permanent. : 
Ammonio-iodide of Cobalt.—Brande observes that “no precipitate is produced in 
