But the progress of this interesting science received a very important impulse from a discovery, which at first scarcely appeared to have any connection therewith. In 1833, Braconnot gave, in the Annales de Chimie, an account of a new substance obtained by the action of nitric acid on starch, sawdust, linen, and cotton-wool. He named this sub stance xyloidinc; it was very combustible, and burned almost without residue. In 1833 Pelouze, ih, the COMptes Rendues, suggested its application iu artillery: He says, " Plunge paper in nitric acid (specific gravity 1500), leave it in for two or three minutes, and wash: a kind of parchment is obtained, impermeable to moisture, and extremely com bustible." 'Dumas, in 1848, proposed the name nitmmidine, and suggested its usb for fireworks. At a meeting of the British association held at Southampton in the year 1846, Herr Schonbein, an eminent Prussian chemist, read a paper on the preparation of explo sive cotton, a substance obtained by acting on ordinary carded cotton by a mixture of strong nitricand sulphuric acids. 'this explosive cotton was afterwards found to be solu ble in ether and the solution so prepared was named collodion by its discoverer, Mr. Maynard, who, in 1848, published in the American Journal of Medical Science the formula for its preparation. This ethereal solution, having a certain proportion of alkaline todidas and iodide of silver added thereto, constituted the collodion first employed by Mr. Archer, who, although deserving the credit of having first arranged a practicable working process with collodion for its basis, without which photography could not have attained its present high position, says, in the second edition of his Manual, "it is due to Le Gray to say that he was the first to publish an account of collodion as it photographic agent;" thus illustrating the candor with which Mr. Archer admitted his claim to be considered the first to suggest its value in photography. Mr. Fallon Borne and Mr. Fry materially assisted Mr. Archer in bringing his experiments to per fection. Although the announcement at the British association in 1846 was to the effect that Schonbein had made cotton as explosive as gunpowder, no particulars were published. In April, 1847, he obtained a patent; but in Oct., 1846, Mr. Thomas Tay lor had published a similar method to that patented. By one of those singular freaks of fortune which sometimes occur, Daguerre succeeded in identifying his name with his process; but Mr. Archer was not so fortunate as to give his name to the which lie invented. A reference to the article COLLODION will show that (bearing in mind that glass perfectly cleaned forms the supporting medium) the sensitive surface is obtained by the conversion of the soluble iodides and bromides in the collodion filet into iodide and bromide of silver by immersion in a solution of the nitrate of that base, and that it is exposed in the camera while still moist with adhere!, Vnitrate, the latentimage so obtained being dereloped with a mixture of protosulphate of iron, acetic acid, and alcohol, fixed with hyposulphite of soda, and varnished.
In the niepeotype or albumen process, glass plates of proper thickness and quality, and perfectly clean, are coated with albumen (q.v.), to which an alkaline iodide has been added. When perfectly dry, they are immersed in a solution of nitrate of silver, when an immediate decomposition takes place; iodide of silver being formed in the albumen film, and nitrate of potash or ammonia remaining in solution. The plate is then freely washed with water, dried, exposed, developed with gallic acid• and fixed with hyposul phite of soda.
A retrospective glance will show the reader that four processes have now been passed in review; and on a little cousiderzttion, it will be seen that one principle pervades the whole—viz., the production of a latent image by the action of light on iodide and bro
mide of silver, its subsequent development by suitable means, and the final removal of the mialtered portions Of the sensitive film by a fixing agent.
Among these processes, that in which collodion is employed has achieved a well merited distinction, and is now so generally employed as almost entirely to exclude the others. Various modifications of this process have been from time to time suggested to meet the exigencies of landscape photography. It has already been stated that the col lodion film is exposed while still moist with adherent nitrate of silver solution; and this method is especially applicable to the taking of portraits, where it is desired to reduce the time of Lxposure to a minimum; but for landscape purposes it is by no means so imperatively necessary to curtail the time of exposure; and as the necessary apparatus and materials for sensitizing and developing a wet plate are somewhat cumbrous for field-work, it was suggisted by the abbe Despratz to wash off the free nitrate from the surface, and allow the film to dry in the absence of light. This is called the '•dry collo dion process." A plate so prepared is, however, much less sensitive to light. A variety of agents hats been used to preserve the film, such as sugar, albumen, malt, tannin, etc.
The practice of photography in the present day is confined almost exclusively to the Posruvu, the NEGATIVE, and the DRY COLLODION processes. Tire first and second differ only in degree, or density of deposit, the image being a having its and shadows in their proper positions, when seen .by reflected light; and a ale; afire, or with its lights and shadows reversed, when examined by transmitted light. The positive is backed with black varnish, and the desired pearly-white deposit produced by develop ment with the protonitrate and sulphate of iron, and nitric and acetic acids. The nega tive is used only as a cliche from which to print positive impressions on paper or other suitable material, and requires a greater degree of density, its high lights being quite opaque, and descending by delicate gradations to its deepest shadows, which should lie represented by clear glass. The negative requires a longer exposure in the camera than the positive, and is generally developed with protosulphate of iron and acetic acid, and intensified with pyiogallic and acetic acids. and nitrate of silver. The Mini or dry process is distinguished from the preceding modifications of the wet process by the com plete removal of the adherent free nitrate of silver, the applicati5n of a preservative agent, and the necessity for adding- nitrate of silver to the developer. There has recently •ome into general use a modification of the dry process, which is likely to supercede all others. It is known as the collodio-bromide or emulsion process, and was introduced by Svce and Bolton of Liverpool in 1804, but attracted little attention till 1871. An emulsion is formed by the addition of bromide of cadmium, and nitrate of silver to col lodion; this is poured on a plate, washed, and immersed in any of the preservative solu tions, and may be exposed wet, or dried and kept indefinitely. Plates so prepared are very sensitive, and possess the great advantage of being capable of development without nitrate of silver; py•ogallic acid, ammonia, and bromide of potass bringing them easily up to printing density.
The following applications of photography may be referred to: