DIALTTIC SEPARATION OF GASES BY COLLOID SEPTA. 435 



The gas consisted of 



2*2 cub. centims. Hydrogen. 



0-3 „ „ Nitrogen, &c. 

 2^ 



The fine silver had therefore occluded 0-211 volume of hydrogen. The metal acquired 

 a beautiful frosted appearance on the surface; and by repeated heating it became 

 highly crystalline and brittle. 



3. The same portion of silver was now charged with oxygen. The occluded gas given 

 off amounted to 



7"5 cub. centims. in thirty minutes. 



7-8 „ „ in one hour. 

 The gas consisted of 



7*6 cub. centims. Oxygen. 



0-2 „ „ Nitrogen, &c. 



^8 



The silver therefore held occluded 0"745 volume of oxygen. This gas, like the 

 hydrogen in platinum, was permanently fixed in the metal at all temperatures below 

 an incipient red heat. It did not tarnish the bright metallic surface of the silver, or 

 produce any appearance suggestive of the oxidation of a metal. 



4. The same portion of silver, after being dissolved in acid, precipitated as chloride, 

 and reduced again, was exposed to atmospheric air at a red heat, and afterwards ex- 

 hausted. The gas given off amounted to 



5"66 cub. centims. in fifteen minutes. 



0-30 „ 



5-86 



Of this gas 5"56 cub. centims., or nearly the whole, proved to be oxygen gas; or 

 the silver held occluded 0-545 volume of oxygen. This silver had been purified from 

 the chloride, and it contained no trace of copper. 



When silver, of British Standard (that is, containing 7*5 per cent, of copper), is ex- 

 posed to air or oxygen at a low red heat, the silver becomes almost black on the surface 

 from oxidation of the copper. Silver wire in this blackened state gave off several volumes 

 of oxygen under the action of heat and a vacuum. Much of the superficial oxide dis- 

 appeared at the same time. It appeared as if the operation tended to the reduction of 

 the superficial oxide of copper, oxygen being liberated, and the copper absorbed by the 

 mass of silver. 



§. A specimen of silver reduced from the oxide, in the form of sponge, which was 

 considered pure, but was not analyzed, occluded 6-15, 8-05, and 7-47 volumes of oxygen, 

 in successive experiments, without any visible tarnish of the surface. Can the attraction 



