[ a83 ] 
SECT. XII. 
Whenever any thing is to be tried, one mnfl 
not begin immediately with the Blow-pipe ; fome 
preliminary experiments ought to go before, by 
which thofe in the fire may afterwards be direded. 
For inftance, a ftone is not always homogeneous, or 
of the fame kind throughout, although it may ap- 
pear to the eye to be fo : The magnifying-glafs is 
therefore neceflary, to difcover the heterogeneous 
particles, if there be any; and thefe ought to be 
Separated, and every thing tried by itfel:^ that 
the effefls of two different things, tried together, 
may not be attributed to one alone. This might 
happen with fome of the finer mic^^ which are 
now and then found mixed with fmall particles of 
quartz, fcarcely to be perceived by the eye. The 
Trapp, {^inQ^xm2,riSchwartzftein) is alfo fometimes 
mixed with very fine particles of Feltfpat {Jfatum 
fcintillans ) or of Calcareous Spar, &c. After this 
experiment follows that, to try the hardnefs of the 
flone in queflion with the fleel. The Flint and 
Garnet-kinds are commonly known to ftrike fire 
with the fteel ; but there are alfo other ftones^ 
though very feldom, found fo hard as to ftrike fire : 
for the adds, mentioned Seft. Ixii. is reduced to lefs than a 
fourth of its original bulk, being exadly of the fame lize with 
the above. It contains two fmall matraffes [Tab. I. fig. 
for making folutions ; a trough [Tab, I. fig. 5.] for walking 
the ore after its being pounded ; and the three liman bottles 
with double fioppers, for the nitrous^ muriatic^ and ‘vitrioUk 
Acids, have their refpe6live initials cut on each. 
Both thefe Pocket-Laboratories, made in the neatcfi manner 
by an ingenious artift, may be had ready furnillied with the 
purefi acid-s &c at the General Office of Buffnefs^ Arts and 
opened for the prefent at No. 98, Wood-Sireetj, 
Cheapfide, and only there^ for very reafonable prices'. 
A 
