142 ME. T. GEAHAM ON THE ABSOKPTION AND 



most attenuated condition, so that the balloon might be cut open and the membrane 

 spread out without shrinking, would be very useful. Instead of depending upon the 

 interior support of sawdust, the membrane could then be stretched over a more con- 

 venient frame to support it, of thin porous deal, of unglazed earthenware, and even of 

 a felted fabric, or several thicknesses of unsized paper supported by a slight frame, 

 so as to form a hollow cavity that admitted of being exhausted of air. The attention 

 of manufacturers of rubber might be advantageously directed to the preparation and 

 proper support of the thinnest possible septa of that material. 



The varnish of rubber which appeared to offer the best septum on drying, was a thin 

 solution of rubber in 200 times its weight of chloroform. Four 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 osmometers, closed with a disk of porous wood or of unglazed earthenware, 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 vulcanized. 



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 purposes. 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 generally 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 

 covered by a small disk of sheet rubber applied warm to the surface. Such varnished 



