The Sensitizer. To render the collodion film sensitive to the light, it must be immersed in a solution formed of nitrate of silver crystals and water. The water must be pure, and the silver free from contamination, used in the following proportions:

Water,..........

36 ounces.

Nitrate of Silver,.................................................................................

3 "

After the crystals are dissolved, add to the solution two grains of iodide of potassium to each ounce of silver used, first dissolving it in a few drops of water, or, what is better, make a solution of iodide of silver, and add it. The iodide mixes with the solution and the potassium is precipitated. Filtration follows, when a few drops of chemically pure nitric acid are added, drop by drop until blue litmus-paper is slowly turned red by it. This solution is usually called the "bath," or sensitizing solution. Placed in the glass bath-holder, it is ready for use.

picture increases with the amount of pyroxylin contained in the collodion; this, however, is a matter of taste: a thick collodion gives stronger contrasts, i. e. the shadows are thinner and the lights more intense, hut a limpid collodion will yield a softer picture, provided the time of exposure has heen sufficiently long. When the contrasts of light and shade are very strong, as in a sunlit landscape, I prefer taking a thin collodion, and by lengthening the time of exposure I obtain a softer picture. - Dr. H. W. Vogel.

136. Most collodions require a faint acidity of the bath, but the utmost care is requisite to avoid adding too much. Putting a glass rod into a nitric - acid bottle, and stirring the bath with it, as some do, is a most clumsy way, and introduces a great deal too much - several drops at once. The best way is as follows: put four ounces of distilled water into a stoppered vial, and drop into it sixty - four drops of pure nitric acid. Each half drachm will then contain exactly one drop of the acid. When you mix your bath with the crystallized nitrate, you may find your collodion work well with it at once. But where fused nitrate is used, acidification is apt to be required. To a twenty-five ounce bath, add a half drachm of the dilute acid, that is, one drop of nitric acid, and try a plate. If it fogs, add another drop and try again. In the last trial which I made, a twenty-ounce bath of fused nitrate required just two and a half drops of nitric acid to make it work right. - M. Carey Lea.

To MakE Iodide of Silver. - Take twenty grains iodide of potassium, and dissolve in one ounce of water; twenty - five grains of nitrate of silver, and dissolve in one ounce of water. A precipitate is formed, which is iodide of silver; wash the precipitate six or eight times and it is ready for use. - Hugh O'Neil.

137. No witch of the Vesuvian Caves ever excercised more diligent care in the preparation of her potent philtres than should the photographer to the preparation of his bath solution. And more over he should be generous about it. It is not enough to merely solution sufficient to cover the plate. It should not expected to bring you good results without a plentiful supply of that, which renders it capable. Therefore, a good large solution is recommended, and is indeed necessary, if the best results and least trouble are an object. And more than this: Do not trust to a single solution, for, like your health, it is liable to break down at any and the most unexpected moment Therefore, always have a second solution in good condition ready for any emergency. This will relieve you from anxiety, and it will save you from many a mortifying failure. Indeed, then: is a great deal of " human nature " in a bath solution.

187. To make a good bath buy a pound of silver. Do not let this startle you, for in a year's time you no doubt buy much more; buy a pound of silver, I say, at once; that is your first step. Oh, that I could impress it upon your mind not to be niggardly in the matter of silver! Dissolve the aforesaid pound of silver in one hundred and sixty ounces of the purest water you can find. Sun the same well, and filter enough into your bath-dish for use; into this drop enough nitric acid, C. P., to redden litmus-paper a trifle. Coat a glass plate well with collodion, and leave in your bath (which latter is perhaps a sixteen-ounce one) over night; then go ahead with your sitters until you begin to perceive that the bath fails to give the good results it did in the beginning, from the introduction of ether and alcohol and iodides; then filter it back into the large bottle of solution, which I take for granted has been sunning, and that you are clean about your fingers in the dark-room generally. Out of your stock solution you may now fill up your bath - dish again, acidifying and iodizing should it need it.

Your first bath should dip at least fifty plates before needing renewing, and your next the same. When in turn this bath fails, proceed as you did with the first one, viz.: filter it back into the stock solution, and fill up your bath - dish again out of the same, this time and ever afterwards omitting the acidifying and iodizing. Proceed in this manner, using the solution and over again, until from continued use the whole bath becomes charged with alcohol and iodides.

At this stage you will find that you have on hand a little over one hundred ounces of solution, testing about forty grains per ounce. I would then advise the following treatment: Heat the solution until at least one - half the water is evaporated, then while still hot drop carefully, a little at a time, ammonia into it until red litmus - paper is turned blue by the same; immidiately set in the sun, when all the impure matter will be precipitated, and may on the solutions cooling bo filtered out. You have then a solution on hand about eighty grains strong, slightly alkaline. To this may be added pure water to bring it up to its former strength. You may now add to same four ounces of silver and forty ouncea of pure water, and you have a bath which, after acidifying, will work for the next six months better than an entirely new one. - C. A Zimmerman.