1887.] MICROSCOPICAL JOURNAL. 173 
tive ; hence the composition of the film requires consideration and the propor- 
tion of coloring matter is important. An approved formula is as follows :— 
Gelatin, 150 grains; white sugar, 20 grains; white soap, 15 grains; dry 
color, 6 grains; water, I ounce. 
The gelatin, sugar, and soap are dissolved in the water and rubbed smooth ; 
the dry color is moistened with water and ground on a paint slab until well 
mixed, then ground with a little of the gelatin mixture, and then added in 
portions to the melted gelatin mass and thoroughly stirred to incorporate all 
well together. The mass is then strained while hot through fine muslin and 
is ready for use. 
The paper may be coated by drawing it over the surface of the melted gel- 
atin mass, or by spreading the mass on the paper, or by coating a glass plate 
with the mixture and spreading the paper upon this. In any case, the paper 
must first be dampened with pure water. Spreading on glass gives the best 
gloss to the carbon surface, and is done as follows :—A c/ean glass plate is 
coated with wax or rubbed with ox-gall and leveled, the warm gelatin mass 
is poured on and quickly spread, as in making dry plates, air bubbles of 
course being avoided or removed. The gelatin soon sets and the damp pa- 
per is then carefully spread upon it, avoiding air-bubbles, and the plate is 
laid aside for half an hour. A knife-blade is then passed under the edges 
and the paper with the gelatin adhering is carefully lifted off, beginning at 
one corner. The paper is then hung up to dry, avoiding dust, and when 
dry will keep indefinitely. To sensitize it the paper is plunged into cold 
water, and as soon as it becomes limp and flat is immersed in a bath com- 
posed of bichromate of potassium 2 parts, water 8o parts, for from 3 to 5 min- 
utes. This sensitizing may be done in dim daylight or by lamp-light, but 
the sensitized tissue must be dried in the dark, in a current of air if possible, 
and free from dust. When dry the tissue is printed by contact under the nega- 
tive in the sunshine for about two-thirds the time required for a print on sil- 
ver paper, as the carbon tissue is more sensitive than silver paper. A pho- 
tometer is of advantage, but a little experience enables one to print the carbon. 
tissue without it, the time for printing on silver from the same negative being 
known. The tissue is not examined during printing, as the image is not vis- 
ible. In printing it is necessary to use a mat or mask which will protect the 
edge for + inch all round from the light. 
To develop the print it is taken from the printing frame and placed in cold 
water until soft and flexible, when it is placed under water, face downwards 
upon the cleaned glass on which it is to permanently remain, excluding air- 
bubbles, and on removal from the water is ‘ squeegeed’ into close contact with 
the glass by means of a rubber squegee, and is set aside for about half an hour 
to drain. ‘The glass with adhering print is now immersed in warm water at 
about 70° F. for a few minutes, then into water heated to about 100° F. and 
gently moved about until the black gelatin is seen oozing out around the 
edges of the paper, when the paper is lifted at one corner and carefully pulled 
off, under water. 
The glass is now rinsed about in the hot water until the soluble gelatin 
is dissolved away, which is quickly done, or it may be held under a gentle 
stream of water at about 80° F., or suspended face downward, in a bath of 
warm water, and allowed to develop by itself. When developed sufficiently 
the prints are immersed in cool, then in cold, water for a minute or two, and 
then immersed for two or three minutes in a three to five per cent. solution of 
common alum, to which, if cloudy, one drop of sulphuric acid to the pint of 
solutionis added. From the alum bath the plates are given a dip into clean 
cold water to rinse them, and are then dried, covered with clear glass or hard 
negative varnish, and mounted for use. 
