With the advance of this beautiful art, there appears to be a progressively increasing desire to produce more artistic results; and improvements are constantly being introduced.

Collodion, as the film upon which to spread the photographic agents, beyond all other preparations, offers, in its exceeding sensibility, beauty of details in the finished pictures, and ease of operating, so many decided advantages, that a separate chapter has been devoted to its consideration.

Collodion is a peculiar preparation, formed by dissolving gun-cotton in ether containing a little alcohol. It is a very mucilaginous solution of a volatile character, the ether evaporating and leaving a film of the utmost transparency behind. It is not all kinds of gun-cotton which dissolve equally well in ether. According to my experience the most easily soluble is prepared by soaking good cotton in a saturated solution of nitrate of potash for some time; it is then, in a moist state, plunged into sulphuric acid with which but a small quantity of nitric acid has been mixed: after remaining in the acid for about a minute, it is well washed with water until no trace of an acid taste is discovered, and then dried at a temperature but very slightly elevated above that of the apartment.

Mr. Archer, to whom, with Mr. Fry, we are mainly indebted for the introduction of this preparation as a photographic agent, gives the following as his processes for preparing gun-cotton:—

" There are two receipts for making gun-cotton, from either of which a good dissolving cotton may be obtained. Several others have been described, but I should only be confusing the subject to attempt to give the whole; and it would be foreign to the limited purpose of this work to do so. The results, however, vary so much with the strength and proportion of the acids used, as to render it extremely difficult to name any one in particular which would entirely succeed under all circumstances. In all cases it is more easy to prepare a cotton which will explode readily, and yet not be at all soluble, than one which will entirely dissolve in rectified sulphuric ether.

Take of dry nitrate of potash 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 chain off the water, and then still further dried by pressure in a cloth ; the fibres of cotton can now be carefully separated, and hung up with pins to the edge of a shelf, or 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, specific gravity 1.450

1 oz. „ sulphuric ditto ordinary.

80 grs. by weight of cotton.

" The fibres of cotton must be well separated, as in the preceding mode. The two acids are first mixed, and the requisite proportion of cotton added as quickly as possible, and well 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, etc, as in the former receipt.

" 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 further, 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".

The substance lignine—which is the true wood of every variety of plant, and has the composition C 38, H 24, O 20—is capable of being converted into a material having an analogous constitution to true gun-cotton. With strong nitric acid lignine combines directly, and forms a substance called xyloidine. This may be prepared by immersing a piece of paper in strong nitric acid, and then washing it well in pure water; it thus assumes the feel and toughness of parchment, and is so combustible as to serve for tinder. The composition of xyloidine is expressed by C 12, N 2, H 8, O 18. Starch dissolves by digestion in strong nitric acid, and on adding water xyloidine is precipitated. There are several other materials of which it may be prepared, and nearly all these substances are soluble in ether, forming collodion.

Gun cotton is a compound of lignine with nitric acid—100 parts of cotton producing 170 parts of gun-cotton.

To Prepare The Collodion

Thirty grains of gun-cotton prepared as described should be taken and placed in 18 fluid ounces of rectified sulphuric ether, and then 2 ounces of alcohol should be added; making thus an imperial pint of the solution. The cotton, if properly made, will dissolve almost entirely; any small fibres which may be floating about should be allowed to deposit, and the clear solution poured off previously to the process of iodizing it. Mr. Delamotte adds a few drops of ammonia to the collodion, which may possibly prevent an acid reaction, which sometimes takes place.