418 Mr. T. Graham on the Absorption and 



containing 42'2 per cent, of oxygen ; barora. 767 millims., therm. 

 23°* 5. For a square metre of surface this is a passage of 2*77 

 cub. centims. of air per minute. 



A small bag, useful for experimental purposes, was constructed 

 of a portion of the same varnished silk, 0*53 metre in length by 

 0'27 metre iu width, which had therefore a surface of 0*143 

 square metre. The varnished side was turned inwards. Between 

 the folds of the silk was placed a double thickness of common felt 

 carpet or a piece of wadding, so as to occupy the interior of the 

 bag. A glass quill tube also entered the bag to the depth of a 

 few inches, and projected as much outside, so as to admit of 

 being connected, by means of a sound adapter tube of Erench 

 rubber, with a Sprengel pump, as shown in fig. 1 (page 412), 

 The edges of the silk cloth were cemented round by caoutchouc 

 varnish, to a depth of 10 millims., so as to close the bag; and 

 care was taken also to cement the glass tube well to the edges of 

 the bag. When the silk bag is exhausted of air, it remains 

 nearly flat, and feels hard like a piece of cardboard. Such an 

 air-dialyzer is further improved by interposing a strong glass 

 flask or bottle, of one or two litres in capacity, between the bag 

 and the pump, so that both are exhausted of air at the same time. 

 The flask must be strong enough to bear the full pressure of the 

 atmosphere without breaking. An auxiliary air-pump, to pro- 

 duce the first exhaustion, cannot well be dispensed with where 

 the space to be made vacuous is so considerable ; the Sprengel 

 tube is brought into action afterwards. The advantage gained 

 by the vacuous flask, and even by the thick wadding placed 

 within the bag, is that they form a magazine in which the dia- 

 lyzed air can be allowed to accumulate for several hours or a 

 whole day, and from which the air may afterwards be drawn 

 quickly by the Sprengel tube for the purpose of experiment. A 

 narrow glass receiver tube, which can be closed by the thumb, 

 may be used to take 5 or 6 cub. centims. for an observation on 

 the inflammation of a chip of wood in the highly oxygenated air. 

 When the proportion of oxygen is under 33 per cent, the wood 

 is not rekindled; but in the ordinary action of this dialyzer 

 the oxygen is seldom found under 40 per cent. The best 

 result is obtained when the exhaustion is within half an inch of 

 the barometric vacuum. When the pressure was allowed to fall 

 to one-half or one-third of an atmosphere, the proportion of 

 oxygen was lessened by 2 or 3 per cent. 



The action of heat and cold on the penetrability of rubber is 

 considerable, as has already been stated. > Operating with the 

 dialyzing-bag described, without any intermediate flask, the vo- 

 lume of air collected in twenty minutes was 6*35 and 6*57 cub. 

 centims. in two consecutive experiments; barom. 760 millims., 



