COLLODION. This viscid sohition is one of the most generally used articles in photogenic operations, and is remarkable as the basis of processes of extreme sensitiveness, and giving pictures surpassing all others for generally good qualities. It is a solution of pyroxyline in ether and alcohol, holding also in solution a' certain quantity of some soluble iodide, bromide, or chloride, or a mixture of them. The principal points to be attended to in its manufacture have now been determined by long experience and many trials. There are two chemical compounds known by the name of pyroxyline, one in which the vegetable fibre operated upon increases in weight about 75 per cent., and a second where the increase is only about 50 per cent. The first is insoluble in ether and alcohol, and not used by photographers. The second is the soluble compound, and upon its preparation very much depends. Though chemically the same it varies in its properties, and modifies the physical and chemical character of the collodion according to the mode in which it has been made. The physical characters which vary are the fluidity, smoothness, contractility, adhesiveness, softness, porosity, cohesion, strength, and transparency of the film formed when the collodion is poured on a glass : the chemical characteristics of the collodion, which are liable to variation with the pyroxyline, are sensitiveness to light, and the density and colour of the image. As to the film, •fluidity, smoothness, softness, porosity, and adhesiveness go together ; and likewise glutinosity, coarse cellular structure, horniness, imper meability to fluids, contractility, and want of adhesiveness go together : and as to the chemical qualities, gre-ater sensitiveness to feeble lights characterizes one kind of pyroxyline and the power of giving dense images another. Two circumstances ars favourable to the production of good physical qualities, viz., a certain dilution of the acids employed and a comparatively high temperature, and the chemical q rties depend upon the temperature, only. When the
most concentrated acids are used insoluble pyroxyline is produced : when the least degree of dilution is formed which, with 120° of temperature, gives the soluble pyroxyline, the collodion has the bad qualities of glutinosity, coarse structure, 8ze., but by gradually in creasing the amount of water, added to clilute the acids, the good qualities grow in degree until a limit is reached where the film in setting becomes short, powdery, and milk white, and partially clis solves in the acids : by stopping a little short of this, and increasing the temperature from 120 towards 170° the physical qualities undergo still further improvement until a limit is also reached in this direction, when the fibre employed again dissolves in the acid. As to the chemical qualities, when the best physical qualities are obtained that acids at 120° will give, it is found that the sensitized film is very quick, but the chemical action is confined to the silver salts, so that the image is weak blue and metallic ; the collodion itself is then a kind of organic matter extremely inert and does not combine in any marked degree with the reduced metal, or take part in the reduction. But if the temperature be raised towards 170° the case is different ; the collodion exerts a decided chemical in fluence in the reduction of the silver salts, and behaves in the same manner as albumen, citric acid &c., in sun-printing. The image is at first red, and is capable of becoming very intense and opaque ; it is more soluble than before in cyanide of potassiam, and in every re action shows that the silver is now combined with organic matter. If therefore, collodion be required for " positives," we select that kind of pyroxyline which gives a collodion with the best physical qualities, but organically inert, for by that means we get a more metallic and brighter tone, and the image being less dense is better modelled and rounded. If negatives are desired we use the pyroxyline made with a higher temperature.