[Sidenote: COLLODION VIALS. COLOR-BOXES.] This shaped vial is made expressly for collodion, to which purpose it is admirably adapted. It has a wide mouth, and is so constructed that the liquid flows clear and free. It is deep, and with a heavy protruding base, to prevent its falling. There are two sizes made at present, one to contain 2-1/2 ounces--the other, 1-1/2 ounce. I generally use the smaller ones, but always keep on hand, and would not be without, a few of the larger size. [Illustration: Fig. 34.] [Illustration: Fig. 35.] Fig. 35 represents a color-box. These can be had of any dealer, completely fitted, with color and brushes for use. =CHEMISTRY.= ~~~~~~~~~~ PRACTICAL HINTS ON =PHOTOGRAPHIC CHEMISTRY.= CHAPTER III. SOLUBLE COTTON--MANIPULATION--PLAIN COLLODION--BROMO-IODIZED COLLODION FOR POSITIVES--DITTO FOR NEGATIVES--SOLUTION OF BROMIDE AND IODIDE OF POTASSIUM AND SILVER--DOUBLE IODIDE OF POTASSIUM AND SILVER--DEVELOPING SOLUTION--FIXING THE SOLUTION--BRIGHTENING AND FINISHING THE IMAGE--PHOTOGRAPHIC CHEMICALS. The chemistry of Photography requires the attention, in a greater or less degree, of every practitioner. It is of the utmost importance, that those who wish to meet with success in the various processes given, should not only be provided with a good selection of chemicals, but also understand the nature of the agent employed. To give a perfectly complete and full list of every agent used would require more time and space than can be given in this work. I shall confine myself to some of the most important, and to such articles as are of the greatest interest to the practitioner. Soluble Cotton. I have, in my practice and trade, adopted the term _soluble cotton_ as the one most appropriate, making a desirable distinction from the article sold as _gun cotton_, they being of a somewhat different nature--gun cotton being the most explosive and least soluble, while the other preparation is more soluble and less explosive. There are two methods employed in the preparation of soluble cotton; one being by the use of nitric and sulphuric acids, and the other with sulphuric acid and nitrate of potash. The last of these I would recommend as being the most convenient for those who require only a small quantity of cotton. Persons experimenting in the preparation of this article should exercise much care and judgment. A good cotton is not the result of hap-hazard operation. The operator should be acquainted, as nearly as possible, with the quality of the chemicals employed, and the proper mode of manipulation.