a. _Variation of Properties in different Samples of soluble Pyroxyline._--The substitution compounds A, B, C, and D differ, as already shown, in the percentage amount of Peroxide of Nitrogen present, and the former are more explosive and insoluble than the latter. But it often happens in preparing Pyroxyline, that two portions of Nitro-Sulphuric Acid taken from the same bottle yield products which vary in properties, although they are necessarily the same in composition. Taking _extremes_ in illustration, we notice two principal modifications of soluble Pyroxyline. The first, when treated with the mixture of Ether and Alcohol, sinks down to a gummy or gelatinous mass, which gradually dissolves on agitation The solution is very fluid in proportion to the number of grains used, and when poured out spreads into a beautifully smooth and glassy surface, which is quite structureless, even when highly magnified. The film adheres tightly to the glass, and when the finger is drawn across it, separates in short fragments, and broken pieces. The second variety produces a Collodion which is thick and glutinous, flowing over the glass in a slimy manner, and soon setting into numerous small waves and cellular spaces. The film lies loose upon the glass, is apt to contract on drying, and may be pushed off by the finger in the form of a connected skin. This subject is not thoroughly understood, but it is known that the _temperature_ of the Nitro-Sulphuric Acid at the time of immersing the Cotton influences the result. The soluble variety is produced by _hot_ acids; the second, or glutinous, by the same acids employed cold, or only slightly warm. The best temperature appears to be from 130° to 155° Fahrenheit; if it rises much beyond that point, the acids act upon and dissolve the Cotton. b. _The physical properties of Collodion affected by the proportions and purity of the Solvents._--Pyroxyline of the varieties termed B and C dissolves freely in a mixture of Ether and Alcohol; but the characters of the resulting solution vary with the relative proportions of the two solvents. When the Ether is in large excess, the film is inclined to be strong and tough, so that it can often be raised by one corner and lifted completely off the plate without tearing. It is also very contractile, so that a portion of the Collodion poured on the hand draws together and puckers the skin as it dries. If spread upon a glass plate in the usual way, the same property of contractility causes it to retract and separate from the sides of the glass. These properties, produced by Ether in large proportion, disappear entirely on the addition of more Alcohol. The transparent layer is now soft and easily torn, possessing but little coherency. It adheres to the surface of the glass more firmly, and exhibits no tendency to contract and separate from the sides.