148 
POPULAR SCIENCE REVIEW. 
all the superfluous gelatine to be expressed at the edges, but 
as soon as the gelatine becomes set, the paper will, on being 
raised from the mould, carry With it the gelatine cast, which 
will be a faithful register of the mould, the heights and 
depths in which being thus translated into colour. As will 
readily be supposed, the preparation of the intaglio mould 
used in this process is an operation of the highest importance, 
for on the delicacy and accuracy of its gradations evidently 
depends the beauty of the finished picture ; and it is in the 
preparation of this mould that the wonderful properties of 
bichromatised gelatine become apparent. 
The particular kind of gelatine employed in the preparation 
of a mould is of more importance than would at first be sup- 
posed. That found by experience to be best for this purpose 
is known as Nelson’s Opaque Gelatine , an ounce of which is 
placed in five ounces of water, allowed to remain until it 
swells, and liquefied by setting the vessel that contains it in 
hot water. To each ounce of this solution must be added 
fifteen grains of bichromate of ammonia, previously dissolved 
in about a drachm of warm water. The mixture should be 
carefully filtered, and kept in a dark place for use. This bi- 
chromatised gelatine possesses some curious properties, the 
nature of some of which will be ascertained from the following 
experiment : — Coat a plate of glass on one side with the 
solution, and when dried, which must be done in a feebly 
lighted place, cover it with a paper containing ordinary printed 
matter on one side, press in intimate contact with the surface 
of the glass by means of a second glass plate, and expose to 
sunlight (through the paper) for a few minutes. On examin- 
ing the plate in a subdued light, those parts on which the light 
was allowed to act, corresponding with the white paper, will 
be found to be deeper in colour than the parts which were 
shielded from luminous action by the black letters. The bi- 
chromate of ammonia has been decomposed by the light, and 
chromic acid has been liberated, which, acting on the gelatine, 
has so modified its nature as to cause it to be no longer 
soluble in water. If now the plate be immersed in cold water 
and quickly withdrawn, it will be found that the parts on which 
the light has been allowed to act are, to some extent, repellant 
of the water. A prolonged immersion in water causes the 
unaltered parts of the surface to swell and stand out in relief, 
those parts in which the chromic acid has been liberated ap- 
parently undergoing no change. 
It will readily suggest itself to a reflective mind that a 
difficulty will exist in the way of securing a series of gradua- 
tions in a photographic negative having their proper effect 
produced when thus attempting to print on the surface of a 
