426 SUMMARY OF CUERENT RESEARCHES RELATING TO 



In the average room having windows on only one wall the side 

 curtains can be left wide apart. In places where the worker is almost 

 surrounded by windows it is of advantage to draw the side curtains so 

 close that there is just room for the observer's head to enter. The 

 telescoping rods supporting the side curtains permits these to be 

 narrowed or widened to suit the circumstances. The top curtain works 

 to or from the microscopist, and is frequently convenient in cutting out 

 the light from the upper parts of the windows. 



Light on the drawing paper is obtained by separating the lower 

 portions of the central curtains from each other and fastening them 

 back. The bulb illuminating the Microscope then throws its light over 

 the right-hand side of the base. A constant intensity of illumination 

 ig in this way assured. 



The fan is a luxmy — possibly an unnecessary one — but in very warm 

 weather, or on days when a few flies persist in maintaining their position 

 at all hazards on top of the writer's head, he has not been at all sceptical 

 as to whether the luxury was unnecessary or not. 



Microscopic Structure of Semi-permeable Membranes and the 

 Part played by Surface Forces in Osmosis.* — F. Tinker thus sum- 

 marizes his researches : — 1. The common precipitation semi-permeable 

 membranes are composed of small precipitate particles ranging from 

 • 1 fj. to 1 /x, these particles being closely packed together. Each of 

 these precipitates is, however, not simple in structure, but is itself an 

 aggregate formed by the flocculation of sub-microscopic colloidal particles. 

 The particles composing the membrane are smallest in the case of 

 copper ferro-cyanide and prussian blue. 2. Precipitation membranes 

 show most of the properties of gels, as ordinarily prepared, both in their 

 method of formation and in the changes they undergo in various 

 solutions. Like ordinary gels they are possessed of great tensile 

 strength, which varies in membranes of different kinds. Their stability 

 in the colloidal condition also varies greatly. But, although they show 

 the physical properties of gels, they have not the same mechanical 

 structure, the membrane being much more closely knit together than 

 the gel proper. 3. The pores in a copper ferro-cyanide membrane 

 range from 8 to 60 yu./x, in diameter, the average diameter being from 

 15 to 20 /iju.. The pore size is too great for the membrane to act 

 osmotically by exerting a selective mechanical blocking action. 4. The 

 order of a series of membranes in pore size is the same as that of their 

 efficiency in semi-permeable membranes. Copper ferro-cyanide and 

 Prussian blue are the most efficient membranes, and they have also the 

 smallest pores. 5. There is a very close connexion between the osmotic 

 properties of a membrane and the extent to which the membrane 

 capillaries are under the control of surface forces. Osmotic effects are 

 probably the result of selective adsorption pheuomena occurring at the 

 surface of the membrane and in the capillaries, the membrane being 

 relatively impermeable to solutes which are negatively adsorbed, but 

 permeable to solutes which are positively adsorbed. 



* Proc. Roy. Soc, xcvi. (1916) pp. 357-72 (6 figs.). 



