Dialy tic Separation of Gases by Colloid Septa. 413 



may often be used with advantage to begin the exhaustion, and 

 to withdraw the greater bulk of the air, if the receiver is large, 

 the Sprengel tube being reserved to complete the exhaustion. 

 The vacuum appears to be as perfect as can be formed in a ba- 

 rometer-tube filled with unboiled mercury, and to come within 

 1 millim. of the barometric gauge. 



The following modifications of the experiment exhibit the dia- 

 lytic action of caoutchouc in its various forms. 



1. India-rubber between double cotton cloth vulcanized. 



This was a common elastic carriage-bag 18 inches by 15. The 

 surface of both sides amounted to 0*3482 square metre. The 

 bag was pressed flat by the hands, and still further exhausted by 

 means of SprengePs tube. After all the contents of the bag 

 were extracted and the collapse complete, the Sprengel tube 

 began again to throw out air in a slow but exceedingly regular 

 manner. A small portion of sawdust, or of sand, introduced 

 beforehand into the bag, appeared to be useful in preventing the 

 sides coming together too closely, but was not essential. The 

 air thus extracted from the bag in one hour amounted to 15*65 

 cub. centims., or sensibly 1 cubic inch; therm. 23° to 24° C. 

 Such dialyzed air, from three successive experiments of one hour 

 each, contained 38, 40*3, and 41*2 per cent, of oxygen, the in- 

 ferior proportion of oxygen in the earlier experiments being no 

 doubt due to a small residue of undialyzed air remaining in the 

 bag before exhaustion. This dialyzed air rekindled glowing 

 wood, so as to illustrate the direct separation of oxygen gas from 

 atmospheric air. For the purposes of combustion, it may be 

 viewed as air from which one-half of the inert nitrogen has been 

 withdrawn. 



It will be convenient to express the permeability of the colloid 

 septum with uniform reference to a square metre of area, and to 

 an hour, or to a single minute of time. Here, for a square metre 

 of cloth, the passage of air amounted to 44*95 cub. centims. (3 

 cubic inches nearly) per hour, or to 0*749 cub. centim. per 

 minute. 



The view which the observation suggests of the nature of such 

 an air-tight fabric is, that it may be truly impenetrable to air 

 when the composition and tension of the air are the same on 

 both sides of the cloth ; but it is penetrable when a vacuum or a 

 reduced state of tension is maintained on one side of the cloth 

 and not on the other. The compression of the air confined in a 

 bag would no doubt have a similar effect, and then the flow would 

 be in an outward direction. But there is no evidence of a porous 

 structure in the varnished cloth. The gases of atmospheric air 

 would pass through actual openings according to the law of 



