13S Observations on the Purity of Standard Gold. 



of bullous ingeniously placed at the botiom of the cru- 

 cible. 



\Vc know that charcoal, when put into close vessels of 

 metal or glass, is not altered, although it becomes red. 

 Theory dictates this, and several experiments confirm it. 

 But the observation of the fact related by the judicious, 

 Heljpt also proves, that in this case the charcoal below the 

 melted silver is decomposed, and continually furnishes 

 elastic fluid j since this excellent chemist found that silver 

 kept over a similar lire, without charcoal being placed be- 

 low it, undulated at the surface, going from trie centre to 

 the edges, and vice versa ; but that in fact it does not bub- 

 ble wiih so much noise : from whence then does the elastic 

 fluid originate in this case? 



Priestley, the founder of the modern pneumatic che- 

 mistry, demonstrates in the plainest manner, what has 

 been since confirmed by several other experiments, that 

 earthen vessels heated until they admit light to pa^s 

 through them, are filters, or rather sieves, which allow even 

 the external air to enter ; that caloric and light penetrate 

 by the bottom of the crucible, and with them air, at- 

 tracted chemically by the charcoal which is inside : its 

 oxygen coming in contact with the charcoal, which is in a 

 state of incandescence, inflames a portion of it, is combined 

 with itself and with the caloric, forming carbonic acid, an 

 elastic fluid, which, by t he uninterrupted action of the fire, 

 augments and acquires a sufficient elasticity to overcome 

 the pressure of a column of seven inches of liquid silver, 

 which is above, and passes through it agitating it violently. 

 The small residue of lead, which is united with and dif- 

 fused over the mass, being put by a continual agitation in 

 contact with the carbonic acid gas, and with the atmo- 

 sphere (the latter and perhaps the former are decomposed 

 by a greater affinity under certain circumstances), is oxi- 

 dated, and by the diminution of specific gravity is con- 

 strained to occupy the upper surface. 



In fact, H el lot saw a kind of yellowish oil rise from the 

 inside of the silver which floated on the top of the crucible : 

 ibis oil was a pure oxide of lead melted and formed by the 

 contact of the atmospheric air, which is continually re- 

 newed. The refiners collect this melted oxide by absorbing 

 it with glass or with sour earth \ this earth being most 

 easily removed from off* the silver which it covers, and the 

 metal then remains limpid and pure. 



On comparing this method with the process of Aaathar- 

 chjdcs, reported here so imperfectly, we may stibpy&e that 



the 



