598 ON DETECTING THE 
obtained. This plan, which Mr. Thompson 
suggests of determining of which of the two metals 
a crust consists, is ingenious, and should be 
adopted in all suitable instances ; but from experi- 
ments which I have made, I am forced to conclude 
that it does not always prove satisfactory—that 
byit indubitable results can only be obtained when 
the crust is very thick, or when its surface is ex- 
tensive, or, in other words, when the quantity of 
metal deposited is considerable. When the crust 
operated upon is arsenic, and only a thin one, or 
sparing in quantity, the colour of the flocculent 
precipitate cannot so distinctly be perceived to 
be yellow as to warrant us in coming to that deci- 
sion, neither can its flocculent appearance be dis- 
tinctly perceived, and we cannot certify that the 
metal in question is not antimony; the plan, 
therefore, is in a great measure liable to the same 
kind of objections which are to be urged against 
the sulphuretted hydrogen, and the sulphate of 
- copper tests ; and I may add, that in this alleged 
instance of only a slight crust, there seems reason 
to doubt whether the result of the silver test alone 
applied as described should be allowed more for- 
cibly to govern our decision, than the distinction 
to be perceived between the appearances of the 
