A Manual Of Photography

260 PRACTICE OF PHOTOGRAPHY. Take of dry nitre in powder, 40 parts Sulphuric acid . . 60 “ Cotton .................2 “ " The nitre, sulphuric acid, and cotton, are weighed in the above proportions, and placed near at hand within reach of the operator, to prevent delay in mixing when the operator has commenced. Then pour the proportion of sulphuric acid into the powdered nitre, stirring them well together for a few seconds with a strong glass rod. Immediately the two are mixed, add the cotton, having previously pulled out the fibres, and mix them well together with two glass rods, in order that the whole of the cotton may come in contact with the nitric acid vapour, which is being rapidly generated from the mixture. This action must be continued for about two minutes ; then quickly remove the cotton with the adhering nitre and sulphuric acid from the basin, with the glass rods, and plunge it into a large quantity of water ; it is to be well washed in repeated changes of water until all the acid and nitre are washed away. The cotton is then collected together, and first pressed between the hands to drain off the water, and then still further dried by pressure in a cloth; the fibres of cotton can now be carefully separated, and liung up with pins to the edge of a shelf, oi any other convenient place, to dry. There is no necessity to use artificial heat, as the small quantity requisite for a few ounces of solution can easily be dried without it. "The next receipt is by certain proportions of nitric and sulphuric acids: Take 1 oz. by measure of nitric acid, sp. gr. 1'450 1 oz. „ sulphuric ditto ordinary 80 grs. by weight of cotton. " The fibres of cotton must be well separated as in the pre- ceding mode. The two acids are first mixed, and the requisite proportion of cotton added as quickly as possible, and veil stirred with two glass rods for not more than fifteen seconds: the gun-cotton is removed from the acids, and plunged into water to undergo the same washings, &c. as in the former recipe. . „ “It will be seen that the cotton is not exposed to the action of the mixed acids, in this last mode, longer than is necessary to saturate the cotton; should the action be continued luiflier, the solubility of the cotton is entirely lost. "Water must not be spared in washing the cotton, for not a trace of acid should be left; the collodion would be injured by any remaining."