150 
POPULAR SCIENCE REVIEW. 
delicate details, and, curiously enough, still remaining un- 
injured after having done so. This discovery at once 
shortened the preparatory process of printing by some days ; 
for the time occupied in producing a perfect mould in metal 
does not now occupy a minute. The metallic intaglio is 
produced in the following manner : — On the flat bed of a 
hydraulic press is placed the gelatine relief, talc side down, 
over which is placed a clean sheet of metal, composed, by pre- 
ference, of a mixture of type metal and lead. A perfectly flat 
plate of steel is placed over this, and the whole subjected to a 
degree of pressure which varies with the hardness of the 
metal employed. For a picture of the size of the portraits 
of Baron Liebig or Professor Huxley, which serve as 
illustrations of the process in the present number of this 
work, a pressure of upwards of fifty tons will suffice to 
impress every detail in the type metal. There is no limit to 
the size of the plate that may thus be produced ; but, in 
proportion as its area increases, so must the pressure also be 
increased. In the mean time, with metal of a suitable degree 
of hardness, the amount of pressure may be approximately 
stated as four tons to the square inch. In obtaining this 
metal intaglio it is of the greatest importance that it should 
be absolutely flat, and for this purpose it is necessary to employ 
two flat polished plates of steel of a thickness sufficient to 
prevent their bending or yielding when in the press. One of 
these should be laid on the bed of the press, and on its face 
should the gelatine mould be placed, the other, as just stated, 
serving to act as a cover. It might be thought that, by passing' 
the two steel plates with their intervening contents between a 
pair of large rollers, pressure would be communicated in an 
equally advantageous manner to that obtained in the hydraulic 
press, and at a less expenditure of mechanical means. Careful 
experiment has, however, determined that the momentary and 
local pressure obtained from rollers will not yield such perfect 
moulds as are obtainable by hydraulic agency. The cause of this 
may be found in the elasticity of the mould, and, possibly, in a 
lesser degree, in that of the metal also. Simple percussion fails 
to yield details in a mould so made ; but even a lesser amount 
of force expended over an appreciable time, say one second, 
will not fail to cause every detail to be impressed in the metal. 
One important advantage in this process is found to arise 
from the fact that the gelatine mould is in no wise deteriorated 
by its having communicated its details to a metallic surface, 
but, where a large number of prints are required, will serve 
to produce several moulds ready for printing, and this, too, in 
a space of time not exceeding one minute for each. 
The process of obtaining prints from the mould is simple. 
