INSTlir.MKNTS AND ( IIKMK'AL TKSTS 



551 



CO! 



ivctly to metal, thus I'bO + CO = Pb + CO 2 , which again may 

 oxidi/ed l.y tin ( ). 1 . The substance to be reduced should be 

 ompletely covered l>\ tin- flame. In a flickering, unsteady flame, 

 \vhere the assay is alternately covered by the R. F. and then 

 po.-ed to the oxygen of the air, there is reduction, then oxidation, 

 (1 in this way the operation may be prolonged indefinitely, in 

 ct good results never obtained. 



Coal and its use. Charcoal is used as a support for substances 

 be tested, either in the O. F. or R. F., and when used with the 

 . F. materially aids the reduction ; as a reducing agent it is mixed 

 irectly with the assay. The best coal is that from light woods, as 

 and willow ; such coal contains a small amount of ash and is 

 better non-conductor. It can be held in the fingers while in use 

 'or a long time without discomfort. It is sawn in lengths of 12 cm. 

 ith a square cross section 2 cm. wide ; coals of this shape are 

 onomical, as all four sides can be used. They should be dry, 

 of cracks, burn without snapping or cracking, and yield a light 

 occulent ash, which is easily blown away. A single piece of coal 

 ill last for a long time, if after each experiment the surface is 

 ither filed or scraped clean with the spatula. In treating an assay 

 n coal a cavity is made with the steel coal borer, near the end of 

 e coal ; the depth of the cavity will depend upon the character of 

 e experiment. If a reduction is required, the hole is made rather 

 eep, 3 to 5 mm., with steep walls. If an oxidizing reaction, the 

 ,vity should be shallow, only sufficient to support the assay. In 

 eating the assay, the coal is held between the first finger and thumb, 

 ith its long axis parallel to the blast, slightly tilted toward the 

 .e, as in Fig. 529. In this position the .coal protects the hand, 



FIG. 529. Position of Assay on the Coal. 



while the draft from the blowpipe will sweep any volatile compounds 

 over the outer end of the coal, where they may collect as coats. 



