Chemical Science. C27 



and the whole heated well for about an hour, will yield the metal 

 chromium*.' 



16. ON THE ABSORPTION op OXYGEN AT HIGH TEMPERATURES, BY 

 SILVER. (Gay-Lussac.) 



Mr. Lucas remarked that fused silver in contact with air absorbed 

 oxygen, which it threw off upon becoming solid. This property is 

 analogous to its power, as observed by Pelletier, of combining, when 

 hot, with double the quantity of phosphorus which it can retain when 

 solid. In the experiments described by Lucas, only small absorp- 

 tions of oxygen occurred, and sometimes none. More certain results 

 are obtained by fusing the silver in a porcelain tube, and passing a 

 current of oxygen gas over it. After twenty-five or thirty minutes of 

 high temperature the current of gas may be stopped, and the heat 

 allowed to fall ; a partial vacuum is soon produced in the tube, be- 

 cause of the diminution of heat ; but at the moment when the silver 

 solidifies, there is an abundant evolution of oxygen gas. 



A simpler and more advantageous process is to throw small quan- 

 tities of nitre upon silver retained in fusion in a crucible. In about 

 half an hour the crucible is to be withdrawn and to be plunged into 

 the water brought under a bell glass. No accident need be feared. 

 There is time to get the crucible under the jar, but immediately after 

 a large quantity of oxygen is disengaged. In one experiment it 

 amounted to twenty-two times the bulk of the silver t. If the metal 

 be allowed to fall drop by drop into cold water, large bubbles of oxy- 

 gen gas will be observed rising through the water, and the metal will 

 acquire a rough and dull aspect. It is to be remarked, that silver 

 containing a little copper absorbs oxygen by its affinity for the latter 

 metal, preserving it from oxidation J. But the purer it is, the more 

 oxygen it takes up ; and a few hundredths of copper prevent the 

 absorption altogether. 



It is of course to this power of absorbing oxygen at a high tempe- 

 rature, and evolving it on solidification, that the phenomenon of 

 vegetation and shooting as it occurs in assaying is to be attributed. 

 It is very difficult to prevent very fine silver from vegetating, but 

 very easy with such as is alloyed with copper, lead, or gold. It 

 is also to this same property of silver of becoming oxidized when hot, 

 that the loss which occurs in cupellation is to be attributed, and the 



* Ann. de China., xlv. p. 1 10. 



{ When nitre is decomposed by heat, it frequently yields a peroxide of potas- 

 sium, which, in contact with water, evolves much oxygen. There is no reason to 

 fear any error on M. Gay-Lussac's part from this cause, but it is necessary that 

 those who repeat the experiments he careful also to avoid it. 



$ The phenomena of polling in the copper works would seem to shew that a 

 similar property belongs, in a slight degree, to copper. 



We have understood from practical men that the effect is easily prevented by 

 charcoal dust being applied over the silver before it is allowed to solidify. The 

 reason of its utility would seem to be very apparent, 



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