1 68 Dr. Percy on the Composition of Beudantite. 



The quantitative results are less satisfactory than the qua- 

 litative. The difference between the proportions of oxide of 

 iron and oxide of lead in the two analyses is considerable; 

 but the following facts may serve in part to explain this dis- 

 crepancy. 



In the first analysis, the crystalline fragments were not 

 selected under a lens, and were mixed with some of the amor- 

 phous brown matrix; while in the second, the pure crystal- 

 line fragments were alone selected under a lens as carefully 

 as possible. Besides, the portions of mineral used in the two 

 analyses were taken from different parts of Mr. Brooke's 

 specimen, and were received by me at different times. 



The differences also between the several determinations of 

 arsenic are considerable; but, irrespective of the probable 

 variation in composition of the several portions of mineral 

 submitted to analysis, it is not to be expected that in analyses 

 of such small quantities of matter as three or six grains, 

 arsenic should be determined with precision. And the same 

 may be said of the determinations of phosphoric acid. It is 

 inferred that the arsenic exists as arsenic acid. 



I wish it to be particularly understood, that I do not pre- 

 tend to present these analytical results as more than approxi- 

 mate; for although I observed every precaution in the investi- 

 gation, yet in the analyses of such a complex mineral as Beu- 

 dantite, precision is not to be expected unless a larger quantity 

 is operated on than I had at my disposal. 



The oxide of lead appears to exist in combination with sul- 

 phuric acid, because in the mineral which has been strongly 

 heated, sufficient sulphuric acid remains to saturate the oxide 

 of lead. Thus 876 per cent, of sulphuric acid was found in the 

 mineral after having been strongly heated, while before it con- 

 tained 1 1*67 per cent. Now 8*76 of sulphuric acid contain 

 5*24 oxygen. The oxygen of the oxide of lead in the first 

 analysis is 1*75, or precisely one-third of that of the acid ; that 

 is, in the ratio which exists between the oxygen of the base 

 and that of the acid in neutral sulphate of lead. In the second 

 analysis it is less than one-third ; still the difference is within 

 the limit which, in the present case, may fairly be assigned to 

 error of analysis. 



Again, the sulphuric acid could only be combined in small 

 proportion with oxide of iron; for otherwise a much larger 

 quantity of acid must have been evolved at the high tempera- 

 ture to which the mineral was subjected in the experiment to 

 determine the loss by calcination previously recorded. 



It may therefore be inferred, that the oxide of lead in Beu- 

 dantite is combined with sulphuric acid. 



