ciple, and no intermediate point is ob- 
ferved between this and the white preci- 
Itis, therefore, te the white pruf- 
itate. 
Soe, what the red is to the green ful- 
phate. The folution of fulphurated hy- 
drogenous gas, if kept in a bottle along 
with Pruflian blue, is decompofed. The 
hydrogen unites with a part of the 
oxygen contained in the Pruflian oxyd, 
seducing this latter to the ftate of white 
pruffiate. This explanation holds good 
when the red fulphate and the nitrate of 
iron are expofed to fulphurated hydrogen. 
The oxyd of iren confumes the hydrogen, 
the fulphur is depofited, and the folution 
gives a green precipitate with alkalies. 
We have by this means a method of 
bringing to the ftate of green vitriol the 
common copperas, as it is fold in the 
Shops. Where a brown precipitate is 
formed, it is a proof that it contains 
copper. 
The hepatic water is not the only me- 
thod that may be employed tobring the blue 
pruffiate to the ftate of white pruffiate. 
The fame effeét is produced if Pruflian 
blue is kept in a well clofed bottle under 
water along with iron or tin filings. 
-It has been mentioned above, that the 
green fulphate of iron does not blacken 
with the acid of galls. This, however, is 
only the cafe when they are firft mixed 
together, for the liquor prefently grows 
dark by abforbing oxygen from the air, 
and blackens from the furface downwards. 
A few drops of oxygenated marine acid 
produce this effect inftantly, and thus it 
appears that the gallate of iron, or com- 
mon ink, contains the metal in the higheft 
fate of oxygenation: and if ink is kept 
in contaét with hepatic water, the black- 
nefs is deftroyed. We thus fee the rea- 
fon why common ink, if frefh made, 
grows darker whilft drying on the paper, 
becaufeithe green vitriol ufually employed 
contains only a fmall portionjof the red 
oxyd mixed with the green. In a word, 
it appears that the property of blackening 
the acid of galis belongs exclufively to 
the oxyd of iron that contains .48 of 
exygen, and therefore is at its higheft 
point of faturations 
‘ 533 
To the Editor of the Munthly Magazine. * 
On Mr. Hume’s Account OF THE 
ORIGIN OF THE IpEA OF NEcks- 
SARY CONNECTION, 
SIR, : 
pe principal means by which Mr. 
~ Hume propofes to ‘ banifh all thar 
jargon, which has fo long taken poffeffion 
of metaphyfical reafonings, and drawn dif- 
gtate upon them,’ is the following: “ all 
our ideas, or more feeble perceptions, are 
copies of out impreffions, or more lively 
ones.’ This he lays down as a general 
rule, and requires thofe who affert that 
it is not univerfal, to produce an inftance 
of fome idea which is not derived from 
any impre{fion. In the mean time, how- 
ever, he takes a precaution which effec- 
tually fecures his principle againft any 
poflible exception, by refolving, that if 
any idea fhall hereafter prefent itfelf, 
which cannot be derived from fome im- 
prefiion, he will confider it as no idea at 
all. <“* When we entertain,” he tells us, 
*¢ any fufpicion, that a philofophical term 
is employed without any meaning or idea 
(as is but too frequent) we need but 
enquire, from what immediate impreffior 
is tbat Juppofed idea derived? And if it 
be poffible to affign any, this will ferve 
to confirm our fufpicion.” ‘ Where we 
cannot find any impreffion, we may be 
certain that there is no 1déa.” 
Having thus eftablifhed his principle, 
he proceeds to its illuftration. The idea 
which he feleéts for this purpofe, and the 
reality of whofe exiftence he thus puts to 
the teft, is that of a caufe. “ We mutt 
enquire,” fays he, ** how we arrive at 
the knowledge of caufe and effect ?”” And 
this, he affures us, perfectly accords with 
his principle; it “arifes from experi- 
ence, that is to fay, it 1s copied from 
our zmmediate impreffions. 
He does indeed acknowledge, that 
“ the particular pawers,”” or caufes, “ by 
which ail natural operations are perform: 
ed, never appear tothe fenfes ;” that is, 
never make any immediate impreffion, and 
that “ he has not by all his experzence ac- 
quired any idea or knowledge of the 
fecret power by which one objeét pro- 
duces the other.” And upon this he 
remarks, agreeably to his principle, that 
“as we can have no idea of any thing 
which never appeared to our outward 
fenfe or inward fentiment, the neceflary 
conclufion feems to be, that we have noidea 
of conneétion or power at all, and that” 
thefe words are abfelutely without any 
MCaRe 
a ee ee a 
