Dialytic Separation of Gases by Colloid Septa. 417 



of chloroform. Pour or five coats of this varnish required to be 

 applied to a surface of wood, or of unglazed earthenware, to form 

 an air-tight envelope. The film appeared to exceed in thickness 

 the rubber balloons, and it dialyzed air less rapidly. But a better 

 result may be expected at the hands of experienced manufacturers. 

 The thin rubber membrane of the balloons was stretched over 

 the ends of glass tubes already closed with a plate of porous 

 stucco — and also over the mouths of small glass bulbs or osmo- 

 meters, closed with a disk of porous wood or of unglazed earthen- 

 ware, and which presented a surface of one-hundredth of a square 

 metre. The membrane of the balloon could only be applied 

 while double ; but after the covering was securely bound to the 

 glass and cemented with fused gutta percha at the edges, the 

 outer coating was torn off, so as to leave only a single thickness 

 of rubber as the dialytic septum. A bulb of the kind described, 

 when exhausted by a Sprengel pump, gave afterwards 16'36 cub. 

 centims. of dialyzed air in two hours, containing by analysis 41 "3 

 per cent, of oxygen, therm. 23° C. ; in the following two hours, 

 17*35 cub. centims. of air, containing 42*6 per cent, of oxygen. 

 This last is at the high rate, for a square metre of surface, of 

 14*46 cub. centims. per minute. 



5. Silk cloth varnished with rubber on one side, slightly vulcanised. 



This is a thin but close silk fabric, much used for waterproof 

 garments. It appears also to be employed, when dyed of a fancy 

 colour, in the preparation of artificial flowers and for other pur- 

 poses. The silk cloth is of a single thickness ; and the coating 

 of rubber, which is of a black colour, appears on one side only. 

 It is a much superior material to the ordinary cotton fabrics, 

 which are double, with the two varnished sides pressed together, 

 and is much more to be depended upon for being sound and free 

 from pores than the " waterproof" cotton cloth. The silk cloth, 

 however, should always be tested by examining air dialyzed by 

 means of it. If the proportion of oxygen falls below 40 per cent., 

 the silk is unsound at one or more spots. These spots may' ge- 

 nerally be discovered by wetting one side of the silk with a 

 sponge and observing where the passage of water is indicated by 

 a visible stain on the other side. The defective spot may be co- 

 vered by a small disk of sheet rubber applied warm to the surface. 

 Such varnished silk, although not the most rapid in its dialytic 

 action, was more convenient in use than any other septum 

 hitherto tried. 



The varnished silk stretched over a disk of porous earthenware 

 (for support) closing the mouth of the small glass bell-jar or os- 

 mometer, which has an opening of one hundredth part of a 

 square metre, gave 10 cub. centims. of dialyzed air in one hour, 



Phil. May. S. 4. Vol. 32. No. 218. Dec. 1866. 2 E 



