254 Determination of Carbon in Graphite. 
To obtain the requisite subdivision we use a weight of the 
silica, from thirty to thirty-five times that of the graphite, adding 
it at intervals, as the materials in the mortar begin to assume the 
adhesive or caked condition which seems to mark the limit of 
the reducing action of the pestle. It is important to bear in 
mind that the success of the oxydating process we are describing, 
is almost wholly dependent upon this preparatory step. In our 
early experiments with the graphite, ground in the ordinary way, 
we found the reaction with the oxydating agents so slow as to 
leave little hope of a satisfactory result. After a digestion of 
twelve hours with the aid of heat, numerous specks of the un- 
changed material were still found at the bottom of the vessel. 
But when, by the process just mentioned, the graphite is brought 
to very minute division, the orydation is completed in from thirty 
to forty minutes. ‘ 
2. Mixture of the Materials—With an apparatus of the di- 
mensions to be described under the next head, six grains of the 
purer kinds of graphite is a convenient quantity for experiment. 
This is triturated with two hundred grains of granular quartz, 
The large excess of bichromate of potash and sulphuric acid 
used in this process, is requisite to ensure a prompt and complete 
oxydation. Any further dilution with water retards the action. 
The sulphuric acid, in the proportion mentioned, is adequate, not 
only to secure an abundant development of chromic acid, but to 
maintain the mixture in a sufficiently fluid state to prevent any 
Serious intumescence while the oxydation is going on. | 
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