28 



put into it another 50 decerns of the phosphate solution, and 5 

 of the acetic mixture, fill the burette to Zero and run into the 

 flask while cold nearly 15 decerns of the Uranium solution 

 and heat. When hot test it with the Ferro-Cyanide, 

 and then keep on adding the Uranium solution two drops at a 

 time, and testing till the faintest chocolate blush is seen. Heat 

 and test again, if the blush is not now seen we must add another 

 drop or two and test again. Note down exactly how many 

 ■decems and tenths it took before the blush remained permanent. 

 We will suppose that 15^- decems were used, that is 15-5 for one- 

 tenth of the whole quantity of phosphate solution, this multiplied 

 by ten equals 155, or the number of decems required for the 

 100 grains of superphosphate. Then as each decern of uranium 

 is equal to "lOQlS of a grain of phosphate of lime made soluble ; 

 155 multiplied by -10915 gives 16-918, which is the amount of 

 :S0luble phosphate in 100 grains of the sample taken, or what is 

 the same thing, the per centage. 



If there was a residue, insoluble in acetic acid, of phosphate 

 of iron, we must add on one and a half per cent, as the average 

 quantity, which has been found by careful experiments of bone 

 earth made soluble, which this phosphate of iron is equal to. 

 Some chemists take no notice of this phosphate of iron, but 

 I believe it is generally thought right to reckon it in as it was 

 originally soluble in cold water. 



To show how near the truth we can get by this plan I will 

 give a few examples of superphosphates analysed as above, and 

 also with the most exact instruments and with very great care. 

 The phosphate of iron was filtered off and weighed in a very 

 delicate balance and calculated into its equivalent of phos- 

 phate of lime made soluble in the exact method. 



No. 1, a mineral manure. By exact method. 

 12-00 soluble phosphate. 



1 .OQ ) soluble phosphate equivalent in the phosphate of iron 

 ji found. 



13-29 



By above plan : 

 12-00 soluble phosphate 

 1-50 added on for phosphate of iron. 



13-50 

 Difference between the two methods : '21, or about two- 

 tenths of a per cent. 



No. 2, a mineral manure. By exact method. 

 18-82 soluble phosphate. 



soluble phosphate equivalent to the phosphate of iron 



1-^9' found. 



20-41 



