CAST IRON. 201 



oxygen, as in organic analysis; the carbonic acid 

 which is produced being collected in a weighed potash 

 bulb (Fig. 22). 



The whole amount of carbon may be determined 

 more accurately by dissolving the iron in water, with 

 5 parts of iodine. The residue is filtered through as- 

 bestos and afterwards ignited in a current of oxygen 

 gas. Or it may be heated in a proper apparatus with 

 six times its weight of bichromate of potassa and an 

 excess of moderately concentrated sulphuric acid, 

 when all the carbon is converted into carbonic acid. 

 The iron may be treated at once in a similar manner. 



Fig. 22. 



Another quantity of the iron-filings is dissolved in 

 dilute sulphuric acid, when the combined carbon is 

 evolved in combination with hydrogen, while the 

 graphite is left undissolved. In this operation, the 

 gas may be conducted through a solution of acetate of 

 lead, when the presence of sulphur is indicated by the 

 precipitation of sulphide of lead. 



The residue insoluble in the acid is well washed, 

 dried at 200, and burnt, as above, in oxygen-gas. 

 From the amount of carbonic acid, that of the graphite 

 is calculated. 



II. SILICON. The residue from the first carbon- 

 determination, which contains all the silicon in the 

 form of silicic acid, is dissolved in concentrated hydro- 



