I N an earlier paper J the writer described methods of preparing and grading collodion membranes for purposes of dialysis. It was there shown that by soaking the air-dried membranes in alcohol-water mixtures of different strengths and subsequently washing in water a series of membranes with a wide range of permeability could be obtained. At the one end of the series (using a grading mixture with a low concentration of alcohol) were membranes which were only slowly permeable to water and in a

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... s degree to the simpler electrolytes, such as sodium chloride, potassium nitrate, &c. at the other end (using a grading mixture with a high alcohol content) were those which allowed a slow diffusion of starch, aniline blue, and other substances which are highly colloidal in aqueous solution. Between these extremes any desired degree of permeability could be obtained. As was pointed out in the paper already referred to collodion membranes of certain grades are directly applicable to osmotic work, as it is possible with certainty to prepare them of such a permeability that they will allow a ready passage of water and at the same time hold back such solutes as cane sugar, copper sulphate, &c. They may thus be used to replace the well-known precipitation membranes (such as the copper ferrocyanide membrane of Traube and Pfeffer), which repeated attempts in this laboratory have shown to be difficult, to prepare satisfactorily. The type of osmometer described in this paper has been used now for some years in the plant physiology practical classes in this laboratory, and has been found to be accurate and simple in manipulation. The following practical notes will serve to show how these osmometers are prepared and used. For convenience in description we shall deal in succession with: 1. Preparation of air-dried membranes or ' thimbles'. 2. Grading and method of attachment. 3. Method of using. 1. The type of membrane most convenient for the present purpose is that known as the thimble, which may be formed either inside or outside 1