I do not pretend to make a collodion which is different in its ingredients from that compounded by others. The only thing is that I am anxious to define the best proportions for making it, and to give a formula which even the most unpractised operator may work by. First, to produce the collodion I always use the soluble paper prepared according to the method indicated by MR. CROOKES, and to which I adverted in "N. & Q.," Vol. viii., p. 252. Take of colourless nitric acid of 1.50, and sulphuric acid of 1.60, equal quantities by measure, and mix them; then plunge into the mixture as much of the best Swedish filtering paper (Papier Joseph is also very good) as the liquid will cover; it must be placed in it a single piece at a time. Cover the basin, and let it remain a night, or at least some hours. Then pour off the liquid, and wash the paper till its washings cease to taste the least acid, or to redden litmus paper. Then dry it. Of this paper I take 180 grains to one pint of ether, and having placed them together, I add alcohol drop by drop, till the ether begins to dissolve the paper, which will be denoted by the paper becoming quite transparent. I have rather increased the quantity of paper to be added, as the after treatment rather thins the collodion. This, when shaken up and completely dissolved, forms the collodion. To sensitize I use two preparations, one prepared with potassium, the other with ammonium compounds; and, contrary to what many operators find the case, I find that the potassium gives the most rapid results. To prepare the potassium sensitizer, I take two bottles of, we will suppose, 6 oz. each; into one of these I put about half an ounce of iodide of potassium in fine powder, and into the other an equal quantity of bromide of potassium, also pounded; we will call these No. 1. and No. 2. I fill the bottle No. 1. with absolute alcohol, taking great care that there is no oxide of amyle in it, as that seriously interferes with the action of the collodion. After leaving the alcohol in No. 1. for two hours, or thereabouts, constantly shaking it, let it settle, and then quite clear decant it off into No. 2., where leave it again, with constant shaking, for two hours, and when settled decant the clear liquid into a third bottle for use. The oxide of amyle may be detected by taking a portion of the alcohol between the palms of the hands, and rubbing them together, till the alcohol evaporates, after which, should oxide of amyle be present, it will easily be detected by its smell, which is not unlike that exhaled by a diseased potato. Of the liquid prepared, take one part to add to every three parts of collodion. The next, or ammonium sensitizer, is made as follows. Take Absolute alcohol 10 oz. Iodide of ammon. 100 grs. Bromide of ammon. 25 grs.