256 On the Manufacture of Optical Glass. [1829. 



glass, which seemed very good in other respects, were fre- 

 quently so discoloured by dark smoky clouds as to be useless. 

 These could not be referred to any impurity which had been 

 left in the materials or had entered accidentally, and, as the 

 platinum was in all such cases altered and injured, 'was at first 

 supposed to be occasioned by some particular action exerted 

 between it and glass at high temperatures. But upon every 

 fair trial to verify such chemical action, the proofs failed, how- 

 ever high the temperature used, or however minutely the 

 metal was divided. At last the cause was discovered. To 

 understand it, it must be known that the platinum tray, with the 

 glass in it, was either placed directly upon the bottom of the 

 iron pan, or, for greater security, with only a plate of platinum 

 intervening ; and that the whole was covered by an evaporating 

 basin turned upside down, forming a sort of inner chamber 

 within the large one. In this confined state the oxygen of the 

 portion of air present was soon abstracted by the heated metal, 

 an oxide of iron being formed in consequence, and at the same 

 time also a portion of carbonic oxide from the carbon in the 

 cast iron. At the high temperature to which the experiment 

 was raised, this carbonic oxide was competent to reduce a por- 

 tion of the oxide of lead in the glass to the metallic state, itself 

 becoming carbonic acid ; but as soon as the carbonic acid so pro- 

 duced came in contact with the heated iron, it was again con- 

 verted, according to the well-known condition of the chemical 

 affinities at these temperatures, into carbonic oxide, and went 

 back to the glass to repeat its evil operation and produce more 

 metallic lead. In this way it was that the glass became sullied by 

 smoky clouds consisting of metallic lead. It was the lead thus 

 evolved, also, that, by alloying with the platinum, had produced 

 the appearance of chemical action always visible in these cases ; 

 and now I knew how to account for the failure of many ex- 

 periments in consequence of the formation of holes in the trays 

 in a manner before quite inexplicable : for in the experiments 

 purposely made to investigate this point, sometimes the glass 

 was darkened only at the surface, the lower part being quite 

 clear and good ; and then, though the platinum tray was fre- 

 quently cut through as with a knife all round level with the 

 surface of the glass, it was quite unaltered below. At other 

 times the superficial stain was in a greater quantity, andjiad 



