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copied by photography, and within two or three hours 
after receipt, pictures could be turned out as fast as the 
printing-press could work them off. A few days ago I 
went over the works to examine the process, and a gentle- 
man, who brought an engraving to the proprietors just as 
I arrived, saw the press printing off very good copies be- 
fore I left, the interval being about two hours, The works 
are at some distance out of London, free from the smoke 
and dust. 
The following is an outline of the history of the pro- 
cess:—Mr. Mungo-Ponton, of Clifton, discovered some 
years ago that if a dried film of gelatine and bichro- 
mate of potash be exposed to light, the film is after- 
wards insoluble in warm water. M. Poitevin afterwards 
noticed that where light had acted upon such a film, it 
took greasy ink just like a lithographic stone, whereas 
those parts on which light had not acted, absorbed water. 
In the attempt to produce pictures on this principle, he 
poured a mixture of warm gelatine and bichromate of 
potash over a lithographic stone, or plate of metal, and 
when the film was dry he exposed it to light under a nega- 
tive. Where the light had acted the film became water- 
proof, and where it had not acted the gelatine swelled up 
like a sponge. This surface of hills and valleys prevented 
him from getting good pictures when he attempted to 
print from it on the lithographic principle. 
Messrs. Tesse du Motay and Marechal tried the pro- 
cess just mentioned, and by carefully selecting their sub- 
jects, choosing those only in which there was little contrast 
of light and shade, they reduced the elevations and de- 
pressions on the surface of the film to a minimum, and 
thus obtained some very fair pictures, but after a very 
few had been printed off, the gelatine printing surface 
broke up. The next man who took up the process was 
Albert of Munich. Before his time, whenever a suffi- 
ciently thick film of gelatine to stand wear and tear had 
been used, the elevations and depressions were so great that 
the film could not be inked. Albert took a plate of glass 
about half an inch thick, covered it with a thick coating 
of bichromated gelatine, and after it was dry exposed it 
all over to light to make it insoluble. Afterwards he 
covered the surface thus prepared with a very thin coating 
of sensitive gelatine, on which the picture was printed 
from the negative. By this process he obtained some ex- 
ceedingly beautiful and perfect pictures, and he is pro- 
ducing them by this plan at the present time. 
Mr. Ernest Edwards took up the process at this point 
about a year ago. He madea thick leathery film at the 
outset by adding alum to the warm gelatine solution. He 
found that films so prepared still retained the lithographic- 
stone-like property ; they will scarcely swell up in water 
at all. They are insoluble, and they resist the wear and 
tear of the printing-press very satisfactorily. 
The working details of the heliotype process are as fol- 
lows. The films are prepared upon large sheets ofaccurately 
levelled finely ground glass, technically known as “ greyed 
glass” ; about 22 inches by 18 inches is a convenient size. 
The surface of the glass is first polished by means of a 
clean piece of rag, with a little solution of wax in ether ; 
the exceedingly thin film of wax thus left upon the glass 
permits the dried gelatine film to come off easily. The 
glass plates after being waxed are levelled, and then a 
measured quantity of a warm mixture of gelatine, bi- 
chromate of potash, chrome alum, and water, is poured 
upon each plate from a jug with a piece of muslin tied 
over its mouth. The temperature of the solution in the 
jug is about 150° Fahrenheit, and after it is poured over | 
the plate it sets in a very few minutes, but it requires a 
much longer time to dry. Curiously enough, until it is 
dry it is not sensitive to light; this fact was found out 
accidentally, for at first this part of the operations was 
carefully carried on in yellow light. 
After the film has set, the plates are taken into a dark 
room to dry. If any of the fumes given off by burning gas 
NATURE 

| Fane 1, 1871 
escape into this room, they act upon the film just as light 
would do, therefore although a gas stove is used to dry the 
plates, the products of combustion are very carefully car- 
ried off. The gas stove used in the works was invented 
by Mr. George, a dancing master at Kilburn. It is a 
closed iron cylinder, into which air is admitted by one 
pipe coming from outside the house, and the products of 
combustion are carried off by another. A third iron air 
pipe enters the bottom of the stove, curves round its sides 
in a spiral, and then emerges through the iron plate form- 
ing the top. Air from outside the house is warmed 
in this spiral, after which it escapes into the drying-room, 
which is kept at a temperature of from go0°to 120°. At a 
temperature of 90° the films take about twenty-four hours 
to dry. As they dry they contract slightly, and thus sepa- 
rate themselves from the glass. These dried films are 
technically termed “skins” ; they are of an orange colour, 
and about one-tenth of an inch thick. The picture is 
printed on them from a negative, and a faintly visible 
image is formed ; when this image is fully out the films are 
removed to a dark room. 
Here each skin is floated in water, and caught upon the 
surface of a thick plate of zinc ; a flat piece of wood, edged 
with india-rubber is then scraped with considerable pres- 
sure over the film, so as to squeeze out all the water be- 
tween the skin and the zinc. As the film still continues 
to absorb moisture, it is thus fixed to the zinc with the 
whole pressure of the atmosphere. After this the zinc 
with its attached film is left for half-an-hour at least ina 
large vessel of water, for the superfluous bichromate of 
potash to soak out, and then the film is no longer sensi- 
tive to light. If the film be thus soaked for several hours, 
or even days, it does not suffer. 
The film, upon its zinc plate, is now ready for the 
printing press. It is damped between each impression, 
just like a lithographic stone. Then it is inked, and the 
best roller for the purpose is found to be one made of 
india-rpbber, backed inside with “ india-rubber sponge” to 
give additional softness. Ordinary lithographic ink is used. 
If stiff lithographic ink be employed, the surface will 
only “ bite” where light has acted most ; if thin ink be 
used, the leathery surface will only bite in the half tones 
of the picture ; hence each picture is produced by at least 
two inkings, and advantage is taken of this circumstance 
to use two colours, and get warm shades in the half tones. 
It is very interesting to see the picture gradually growing 
under the inking process. By this method double- 
printing is executed with a single pull at the press. 
Ordinary Albion hand printing presses are used. 
The negatives worked fromin this process have to be 
“reversed,” and they may either be reversed at the time 
they are taken, or afterwards. In the former case, instead of 
the lens of the camera being pointed direct at the object 
or picture to be photographed, a mirror, silvered on its 
front surface, is interposed at an angle of 45°. Another 
method of reversal is to take an ordinary unvarnished 
negative, and coat it either with a solution of india- 
rubber, ora solution of gelatine and alum. When the film 
is dry the plate is accurately levelled ; it is then coated 
with a pool of collodion as thick as it will hold, and this 
collodion is then allowed to dry. Next the film is cut 
through with a penknife near the edges of the picture, 
and the plate is placed in water, where the negative soon 
floats off the glass, after which it is dried between blotting 
paper. The flexible negatives thus obtained are very 
durable, except when bad india-rubber is used in reversing 
them. 
When a batch of pictures has been prin'ed from any 
particular skin, the film is taken off the zinc plate, and 
put away until wanted again. Mr. Edwards says the 
skins will stand a vast amount of wear and tear, and 
he showed me one from which he said 1,500 pictures 
had been printed, the last impression being as good as 
the first, and the skin ready for further work if necessary. 
