C. G. Wheeler on @ Process of Elementary Analysis. 35 
volume of freshly ignited oxyd of copper. It is important that 
the mixture be as uniform as possible in order that the supply 
of oxygen may be easily regulated. Then follows about two 
inches of oxyd of copper, and thereupon a weigiied portion, from 
0°2 to 03 gram, of oxalate of lead which is likewise thoroughly 
mixed with oxyd of copper. This salt is the substance I find 
the most convenient from which to obtain a known amount of 
carbonic acid gas. 
. L adopt oxalate of lead in preference to other substances that 
have hitherto been employed in organic analysis for evolving 
carbonic acid, as more completely free from thé various practical 
objections presented by the latter, and in particular by the follow- 
ing, viz: carbonate of magnesia (magnesite), carbonate of man- 
ganese, carbonate of copper, bicarbonate of soda and oxalic acid. 
_ The use of these substances is impracticable, as they either fail 
to give the theoretical percentage of carbonic acid on being 
heated, partially decompose when exposed to the air in a moist 
condition, yield also water with carbonic acid, are too hygro- 
Scopic, or present other difficulties that render their use in this 
process inexpedient. Carbonate of lead is less objectionable 
than the above mentioned, and it is mainly on account of the 
greater amount of carbonic acid furnished by the oxalate that I 
prefer it. 
I prepare the latter salt by adding to a solution of acetate of 
lead a slight excess of oxalic acid, and thoroughly wash the 
precipitate obtained by decantation. It has the formula PbO, 
’20,, and yields on gentle ignition with oxyd of copper pre- 
cisely two equivalents of carbonic acid. s a mean of severe 
nearly identical results I obtained 29°83 p. c. of carbonic acid 
Instead of 29°81 as required by theory. A greater amount 
of this salt than 0:3 gram being never usefl, the maximum 
error, therefore, possible in a carbon determination woul 
0:00001643 gram. das 
