166 Messrs. Calvert and Johnson on the Chemical Changes which 
powder, either by pulverization or by means of a file, and then 
to burn the carbon under the influence of a red heat by a slow 
eurrent of pure and dry oxygen gas. The apparatus which we 
used was the following :— 
a, a flask containing a mixture of chlorate of potash and oxide 
of copper, which by gently heating gave off a regular current of 
oxygen. 
b, a bottle containing a concentrated solution of caustic pot- 
ash, so as to retain any chlorine or any oxygenated compound of 
this gas which might be produced. 
c, a tube full of pumice-stone moistened with solution of 
caustic potash, and employed with the same view as the last. 
d, a U-tube filled with pieces of solid melted caustic potash, 
also used for the same purpose. 
e, a bottle containing sulphuric acid for retaining any moisture 
which might accompany the oxygen gas. 
f, 2 porcelain tube in which was placed a small porcelain dish 
containing the pulverized iron. 
g, a tube filled with small pieces of pumice-stone . moistened 
with sulphuric acid, with the view of retaming any moisture. 
h, a Liebig-tube full of concentrated solution of caustic pot- 
ash, to determine the amount of carbonic acid produced by the 
combination of the oxygen with the carbon of the iron. 
i, a small tube with fragments of caustic potash, to retain any 
trace of carbonic acid which might not be absorbed in the Liebig- 
tube. 
To render the absorption of the carbonic acid complete, it is 
necessary to conduct the operation very regularly and slowly ; 
therefore about two hours are required to burn all the carbon 
existing in about 3 grammes of iron. 
By this method two analyses of the same sample seldom pre- 
sented a greater difference than 0:05. We also took the pre- 
caution to dissolve the oxide of iron obtained after combustion, 
in order to sce that no hydrogen gas was given off, and conse- 
quently that no metallic iron remained. 
