Why does the high temperature render the collodion more apt to take part in the reduction of the silver, and to combine with it ? Cotton fibre is nearly pure lignin, and the tendency of acids is to cause lignin to pass successively into starch, gum, sugar, glucose, saccharic, and other acids, which have each a greater reducing power than the preceding member of the series, though they resemble each other extremely in their ultimate composition. Heat favours this action of acids upon these organic substances, and the probability is, that when vegetable fibre is submitted to a comparatively high temperature in the making of pyroxyline, some change of this kind may take place, in a greater or less degree at the same time that the lignin is changed by the acid as before. The acid we may suppose first changes the fibre into an isomeric body, more analogous to gum, for lignin and gum are both represented by Hso and then the further substitution of NO, for a certain number of atoms of hydrogen, completes the change into pyroxyline. This view is confirmed by an experiment, in which a remarkable collodion was produced very fluid, with 30 grains of pyroxyline to the ounce of mixed ether and alcohol. Swedish filtering paper was allowed to lie half a minute in a mixture of equal volumes of sulphuric acid and water, until parchmentized, and then used, as had been previously done, by Mr. Hardwich, for conversion into pyroxyline : the weakest acids, and the highest temperature were resorted to, that it was pos sible to try. The parchment dissolved for the greater part, but after 2 minutes immersion, the acid, the vessel, and all were plunged into water. Scraps of yellow looking parchment were collected, washed and dried ; collodion made with this showed negative qualities in the very highest degree. The image was exceedingly red at first, and could be intensified to almost any extent. Like all organic matter, it was also found to interfere decidedly with the sensitiveness of the film to light, and this more and more the longer the col lodion was kept. This really photographic action of collodion cannot be disputed, though it has generally been said that it is inert, and that its good qualities are entirely mechanical. It may be considered as demonstrated, that where intensity of image is de manded, organic matter must take part in it. It might be suppose,d that this pyroxyline dissolving 30 grains to the ounce, was a different compound fi.om the ordinary kinds : it might have been, but dif ferent kinds of pyroxyline have been made dissolving from 1 to 15 grains, which were made without loss, and all of which increased in weight about 50 per cent. in the making, so that they were pro bably all of the same compobition.
With respect to the solvents of the pyroxyline, the ether and alcohol, a similar regard must be had to the kind of collodion re quired. For collodion positives it is necessary that they should both be free from organic matter, or they will form a collodion not sensible to feeble lights, which will show itself in pictures wanting half tint in the shadows, and overburdened with material in the lights, in fact, too negative. Careful distillation from caustic
potash is necessary in this case, and also a prudent choice of alcohol in the first instanc,e. It is extremely difficult to decompose the grain oil, and other matters occuning in some alcohols made from roots and seeds, and they should be rejected for photographic purposes. For negatives it is not essential that the spirits should be perfectly pure so long as they are neutral, and good rectified ether and alcohol will answer well : to make negative pyroxylines, proceed thus :— Take one ounce of old linen or calico that has been many times washed. Boil it in weak caustic potassa until it ceases to give colour to the solution, and then let it remain as long as convenient in the solution. This modifies it in the same way as linen and cotton fabrics are modified by alkali, when they are intended to be bleached ; that is to say, they are rendered more susceptible of oxi dation. Then to 10 fl. ounces of sulphuric acid, S. G. 1.84, in a warm basin, add 12 fl. drachms of water, and next 10 fl. ounces of nitric acid, S. G. 1.45. The temperature will probably rise to 140°, or thereabout. If it does not, place the basin in a deep dish con taining boiling water until it does, and if it goes beyond that tem perature, wait until it cools to that degree. Take the basin if neces sary from the hot water, and immerse in the mixed acids the ounce of linen previously washed from the alkali, dried and torn into strips of 6 or 7 inches long, and 2 or 3 wide. When the linen is immersed, the temperature will probably rise to and if it does not, replace the basin in the hot water. Keep the whole at a temperature of 150° for twenty minutes moving the linen so as to secure uniform reaction. Then pour off the acid, plunge the basin and its contents into a very large vessel of water, and stir it up vigorously. Allow the pyroxyline to settle, and immediately pour off the water ; squeeze the cotton and place it in another vessel of water; continue this washing until no acidity is shown by litmus paper, and then allow the pyroxyline to soak some hours in water. Lastly, squeeze well in a clean dry cloth, and spread the pyroxyline in a warm room to dry. It will be found necessary with almost every fresh sample of nitric acid, to try by experiment on small quantities, whether the above proportion is the weakest possible mixture of the acids that will act without dissolving much of the linen. If none at all is dis solved, it is almost certain that more water may be added. The pyroxyline ought to dissolve 6 or 8 grains to the ounce, and give perfect fluidity and a structureless film.
To make positive pyroxyline. Use cotton wool instead of linen. Keep the temperature about and allow the acids to act for 5 or 6 minutes : it should dissolve 4 grains to the ounce.