" The first three or four samples are called " Pyroxyline," and the last three or four " Xyloidine." But it will be seen that this nomenclature is imperfect, for it does not include the middle varieties, which are those with which we are concerned in Photo graphy, viz : Nos. 5, 6, 7 and 8. We will call them photographic gun-cotton.
" The first samples of pyroxyline are highly explosive. Place a small tuft upon the hearth, and apply to it the end of a red-hot piece of iron wire. It instantly goes off puff,' without smoke, and le,aving no ash. The last varieties of xyloidine are merely combustible, and not explosive.
" Let us next examine the nature of the solutions made with samples between Nos. 5 and 10.
" Pour a few drops of No. 5 upon the finger, so that it may run anuid both ways. It dries quickly (producing a sensation of cold), and, when dry, contracts strongly, looking like a piece of goldbeater's skin, stuck tightly round the finger. This is the hard contractile Nfiodion. Its use should be avoided in photography.
" Now pour a few drops of No. 10 upon the finger. This also dries ' " " when dry does not contract like the former, and instead of being transparent, is semi-opaque, or " opalescent," or " papy raceous, ' looking like a piece of tissue paper stuck round the finger. This is also a kind of collodion to be avoided in photography.
" The proper variety of gun-cotton for photographic purposes lies between these extremes.
" In order to try which is the best, pour a little of each solution upon a clean glass plate. But in order to make the experiment fairly, wait a day or two, until the floating particles in the collodion have settled to the batom of the bottle ; for you cannot properly filter collodion without a special apparatus.
" Examine the films before a strong light, with the help of a magnifier.
" Film No. 5 is not only hard and contractile, but shows struc ture, being covered with wavy marks, or lines.
" Films Nos. 6 and 7 are much better and are nearly structureless ; No. 6 is the best.
"Film No. 8 begins to get slightly opaque.
" In films Nos. 9 and 10 the opacity increases.
" No. 6 is therefore the best collodion, and on adding a little alcohol to it, the appearance of structure in the film altogether vanishes. It adheres tightly to the glass, without contracting, and cannot easily be washed off.' The proper strength of the nitrosulphuric acid, determined from the above experiments, does not agree exactly with the formula given in symbols in a former paragraph, and which is that of Mr.
Hadow ; but some latitude is allowable in that formula, and the best proportions are purely a matter of experiment.
Before putting the cotton into the acids it should be pulled out into very thin flat pieces.
Instead of using mixed acids, a mixture of sulphuric acid and nitre may be employed. The ingredients must be pure, and the nitre finely pulverized and dried.
Mix together— Water . fluid clrachms.
Sulphuric acid . 12 „ And pour it upon Pure nitre . . 600 grains.
Stir it well until it ceases to effervesce, and forms an even pasty mixture free from lumps. This mixture must be kept at a tempera ture not lower than 135° and used at once, as it solidifies on cool ing. The cotton must 'be kneeded in the mixture for about 10 minutes. This plan of making pyroxyline is not nearly so good as with the mixed acids.
In using commercial acids of unknown strength, the exact quantity of water to be added is a matter easily determined by two or three experiments.
The theory of pyroxyline is as follows :—Lignin is composed of carbon, oxygen, and hydrogen. When acted on by nitrosulphuric acid, from 3 to 5 equivalents of hydrogen are removed, and their place supplied by peroxide of nitrogen, by a process called in chemistry " substitution." The only effect of the sulphuric acid appears to be, to prevent the nitric acid from dissolving the pyroxy line ; for sulphuric acid has a strong attraction for water, and pyroxyline is soluble in dilute though not in strong nitric acid. The theory expressed in symbols is 0 + = C 0 + HO. Lignin + nitric acid = pyroxyline + water, in which expression the number of atoms involved in the change is, for the sake of simplicity, omitted.
Pyroxylin being a substitution compound is very unstable. Bottles containing it are frequently filled with red fumes, and by its decompo sition in the form of a collodion film, oxides of nitrogen are given off which destroy the picture. It should, therefore, be dissolved in ether as soon as made, and collodion pictures should always be varnished.
Pyroxylin is sometimes made from Swedish filtering paper ; but the plan is liable to the objection that paper may contain mixed fibres of different kinds, which would be unequally acted on by the acids, at the same time that the fibres in the interior of the paper are not so readily acted on as those at the surface. These constitute weighty objections to the use of filtering paper.
For further information on this subject see " Collodion."