[Sidenote: PLAIN COLLODION.] To dissolve the soluble cotton (pyroxyline), and form plain collodion, proceed as follows: Take of Sulphuric ether (concentrated), 10 ounces Alcohol, from 90 to 95 per cent., 6 " Soluble cotton enough to give the solution a consistency such as will allow it to flow evenly over the surface of the glass, and impart to it quite a thick and transparent coating. If the coating is opaque, the cotton has not been properly prepared, the acid mixture has been too weak. _Remarks._--It is desirable for every operator to use chemicals of uniform strength, and the better method to adopt is to employ those purchased from some one respectable manufactory, and not take those furnished by irresponsible and unconscientious parties. At least one-half of the failures experienced by beginners is from want of good chemicals. It is not economy to purchase a _cheap article_. _Alcohol_ is an article that can be procured in almost any small village in the United States, and is in general fit for collodion purposes. I have used 88 per cent, in the above proportions, also the intermediate varieties to 98 per cent., and have been quite successful; but feel convinced that the ordinary 98, as marked (which usually stands by actual test 95 to 97 per cent.), is preferable, except in cases where water is employed in dissolving the iodizing salts, when I would use fully 98 per cent. Before concluding the subject on plain collodion, I will introduce the account given by Mr. E. A. Hadow of his interesting and valuable experiments, as published in _Humphrey's Journal_, Vol. VI, page 18. "Having obtained good collodion-wool, the next point of inquiry was with regard to the solvent; to ascertain whether the addition of alcohol beyond what is necessary to cause the solution of the gun-cotton in ether, were beneficial or otherwise. For this purpose ether and alcohol were prepared perfectly pure, and mixtures were made of 1 of alcohol to 7 of ether, 2 to 6, 3 to 5, 4 to 4 and 5 to 3. In one ounce of each were dissolved 6 grains of gun-cotton and 4 grains of iodide of ammonium (iodide of potassium could not be employed, since it requires a certain amount both of water and alcohol to keep it in solution); they were then compared, using a 35-grain solution of nitrate of silver, both by pouring on separate glasses, and likewise by covering two halves of a plate with two samples, as in examining the gun cottons, thus placing them under the same circumstances during the same time; in this way the effect of adding alcohol was very clearly perceived, since the difference between the collodions was much greater than could have been anticipated.