100 Mr. Smithson on [Feb. 



Supposing that the iron and copper exist in the mineral in the 

 state of peroxide, and that the weights of their atoms are to each 

 other respectively as 40 to 80, it will be impossible to reduce the 

 martial arseniate of copper to a probable definite compound ; for 

 it will appear by calculation that the nearest approximation is 

 5 atoms of oxide of iron and 2 atoms of oxide of copper ; it 

 seems, therefore, more likely that the skorodite is a peculiar 

 arseniate of iron, differing not only in form, but in composition, 

 from the cubic arseniate of iron; and it will follow, if this be 

 admitted, that the martial arseniate of copper is a mixture and 

 not a compound of arseniate of iron and arseniate of copper : 

 this supposition will, perhaps, be considered the more probable 

 when it is remembered that the cubic arseniate of iron contains 

 9 per cent, of oxide of copper. It is also to be observed, that 

 M. Chenevix inclines to the opinion that it is a mixture of the 

 two arseniates ; and lastly, there appears to be no reason why 

 there should not exist several varieties of arseniate of iron, which 

 is well known to be the case with arseniate of copper. 



Article IV. 



On some Compounds of Fluorine. By J. Smithson, Esq. FRS. 

 (To the Editor of the Annals of Philosophy .) 



SIR, Jan. 2, 1824. 



When numberless persons are seen, in every direction, pur- 

 suing a subject with the utmost ardour, it is natural to conclude 

 that their labours have accomplished all that was within their 

 reach to perform. 



It must, therefore, in mineralogy be supposed, that those sub- 

 stances whose abundance has placed them in every hand, have 

 been fully scrutinized, and are thoroughly understood ; and that 

 if now to extend the boundaries of the science it is not indis- 

 pensable to explore new regions of the earth, and procure mat- 

 ters hitherto unpossessed, it is yet only to objects the most rare, 

 the most difficult of acquisition, that inquiry can be applied with 

 any hope of new results. 



A want of due conviction that the materials of the globe and 

 the products of the laboratory are the same, that what nature 

 affords spontaneously to men, and what the art of the chemist 

 prepares, differ no ways but in the sources from whence they are 

 derived, has given to the industry of the collector of mineral 

 bodies an erroneous direction. 



What is essential to a knowledge of chemical beings has been 

 lsft in neglect ; accidents of small import, often of none, have 



