This solution is made in the same manner as in the foregoing article, substituting the iodide of potassium for the bromide--no bromide being used in this preparation. The yellow precipitate in this case will be re-dissolved and taken up in the solution: it may require more than one ounce of pulverized iodide of potassium to effect this, but it may be added in excess, so that the solution shall contain a quantity in powder. Developing Solution. [Sidenote: DEVELOPING SOLUTION.] Protosulphate of iron, 3 ounces. Rain or distilled water, 1 quart. Put these into a quart bottle, and shake until the crystals are all dissolved, and this can be kept for a stock bottle, and when wanted for use pour into another bottle. Of the above solution, 5 ounces. Acetic acid (No. 8) 1 " Chemically pure nitric acid 20 drops. Shake this mixture well, and filter through a sponge, and it is ready for use. I file a mark in this bottle indicating five ounces, and another for 1 ounce: this will save time in mixing the solution. _Remarks._--In my recent tour of the United States, I found it difficult to obtain a good article of protosulphate of iron, and in its stead I used the common copperas, such as I could find almost in any store. I employ from one-fourth to one-half more than the quantity given above. If it looked a clear green, and free from a white or brownish powder, about one-fourth addition: _i. e._, four ounces, instead of three, as given above. If the solution in the stock bottle is not wanted for a week or more, a few crystals of the protosulphate of iron should be added, as it decomposes, and the strength is depreciated. There is quite a difference in the strength of the acetic acid as sold by out country druggists, and the operator should be sure that he has No. 8, to which quality the above proportions are adapted. I never have employed the developing solution but once, but can see no objections to use it for a number of glass plates, but it should be filtered every time before using. The quantity of nitric acid may be increased, so long as a proper proportion is preserved with the strength of the bath. The effect of this addition of acid will be to brighten the impression; but if carried too far, the reduction (developing) will be irregular, and the harmony of the impression injured. Fixing Solution. [Sidenote: FIXING SOLUTION.] Water, 8 ounces. Cyanide of potassium, about 1 drachms