186 On the Composition of Oceanic 



D. Estimation of Insoluble Residue, Fe 2 O 3 , A1 2 O 3 , and 

 Co 2 O 3 . 2'o88o grammes were treated with strong hydrochloric 

 acid in a covered beaker, the mixture evaporated to dryness 

 over a water-bath, the residue moistened with a few drops of 

 strong hydrochloric acid, taken up with water, and the solution 

 filtered. 



The insoluble residue was heated over the water-bath with a 

 strong solution of carbonate of soda. The heating was carried 

 on for about forty-five minutes, water being occasionally added 

 to keep the solution of constant strength. The solution was 

 then decanted through a filter, more carbonate of soda solution 

 added to the insoluble residue, and the same process repeated. 

 This was done altogether three times. What remained un- 

 dissolved was washed, ignited, and weighed. It was found to 

 weigh 0-5790 gramme, that is, 27*73 per cent., consisting of 

 silica and silicates, which resist the action of sodium carbonate 

 solution. 



The silica which had dissolved in the sodium carbonate was 

 precipitated from it by hydrochloric acid as usual, filtered, 

 ignited, and weighed. It weighed 0-11928 gramme, that is, 

 5-71 per cent, of soluble silica. 



The filtrate from the insoluble residue was treated with 

 excess of carbonate of baryta to precipitate the iron and 

 alumina. The precipitate was filtered off and redissolved in 

 hydrochloric acid, the baryta present precipitated with sulphuric 

 acid and filtered off, the filtrate made up to 250 c.c. volume, 

 and two estimations of Fe 2 O 3 + A1 2 O 3 made. 



100 c.c. (= 0-8352 gramme of substance) gave 0-03368 

 gramme, Fe 2 O 3 + A1 2 O 3 , i.e., 4-03 per cent.; 75 c.c, (=0-6264 

 gramme of substance) gave 0-02568 gramme, Fe 2 O 3 + A1 2 O 3 , 

 i.e., 4-09 per cent. The two precipitates of Fe 2 O 3 and A1 2 O 3 

 were united and fused with bisulphate of potash. An insoluble 

 residue of silica, weighing 0-00898 gramme, was found. This 

 represents 0-61 per cent, of silica in the nodules, which, when 

 added to the previously found 5-71 per cent., gives altogether 

 6-32 per cent. 



Also, subtracting 0-61 from 4-06 (the average of the iron and 

 alumina estimations), we find 3-45 per cent, of Fe 2 O 3 -f A1 2 O 3 . 



