98 
ON THE DOUBLE SALTS OF TROPf. 
in drying, and I only obtained from a pint of solution about 
one scruple in a perfect state, and about as much more in a 
slightly imperfect* condition. The perfect salt is in lamellae 
of a ruby color, and is soluble in water ; ammonia precipi- 
tates oxide of iron from its solution, and the affinity between 
the oxide and acid is so trivial, that a little elevation of 
temperature is sufficient to overbalance it ; in its dry state 
it will bear considerable heat without alteration ; before 
examination, I held it some minutes very near to a common 
fire. The twenty grains of slightly imperfect salt were 
made up to twenty-five, and heated to redness; after fifteen 
minutes it was found to weigh eight grains: the remainder 
was preserved, as I did not like to part with the very little 
that was with difficulty obtained. 
It appears to me that the nature of the precipitate soluble 
in alkalies, spoken of in cases 3 and 4, has been mistaken, 
and that they have been confounded with the oxide of iron 
itself. I some while ago met with an observation, but have 
failed to discover it since, to this effect — The further ad- 
dition of oxide (meaning addition after solution had ceased) 
causes the separation even of that portion which had been 
previously dissolved.''* A writer in the Pharmaceutical 
Journal, vol. i. p. 595, says, " Let the oxide be added in 
excess. 5 ' M. Beral also directs that " rather more oxide 
should be added than the acid will dissolve." Whatever 
may be the value of these remarks, or how much they may 
conflict with each other, it is not my present business to in- 
quire ; I refer to them, because individually, and together, 
they do seem to imply that the fact of the oxide combining 
after solution had ceased, has been overlooked ; and possi- 
bly to the prevalence of this oversight we may attribute 
much of the erroneous impression which so generally ob- 
tains. On a further comparison, I find that more of these 
* The imperfection consisted in minute opaque dull spots, arising from dis- 
engagement of acetic acid and deposition of oxide. 
