1816.} Scientific Intelligence. 478 
present construction. He informed me that the sole object was to 
economise the fuel as much as possible. It is not the shallowness of 
depth of the boiler that produces this effect ; but the making of the 
boiler of sueh a shape that the air which passes through the fire 
shall be robbed of almost all its heat before it can make its escape: 
Mr. Watt assures me that this object is very well attained by the 
present construction. I have sometimes thought that if the boilers 
were covered over with some bad conducting substances, they might 
perhaps require rather less fuel. But probably there is a good 
reason for the present practice, though I am not acquainted with 
it.—T. ' 
VU, On Mr. Donovan's Essays, and the Mode of procuring pure 
Silver. 
(To Dr. Thomson.) 
SIR, 
I observe in the last number of the Philosophical Magazine that 
some reflections have been cast on the Royal Irish Academy for 
their reception of errors said to have been made by Mr. Donovan 
in one of his essays. He has apologised for that body, by answering 
that his opponent mistook the paper, and that they gave their prize 
to another of his productions. As I find in the same magazine that 
this will soon be published, and as the opinions which it contains are 
of astartling novelty, it is necessary to state that the Academy did not 
give Mr. D. tie prize. In fact none of its members in the slightest 
degree admitted his conclusions ; but in consideration of the labour 
which was apparent, he was allowed a portion of it to encourage his 
future efforts. I think it right that this should be known, as we are 
apt to jadge of scientific bodies by their Transactions ; at least I 
have often heard the Secretaries of the Royal Society arraigned for 
suffering particular articles to appear in the Transactions. 
I see in the same publication a process proposed for purifying 
silver, by the same Gentleman, of which I request your opinion. 
It is generally thought that silver precipitated by copper always 
contains some of this metal, and I think it very unlikely that it can 
be removed by ammonia. Cautious crystallization will purify nitrate 
of silver considerably, as the nitrate of copper is much more soluble 
than the other; but Gay-Lussae has given a process which appears 
very plausible. He asserts that oxide of silver has a stronger affinity 
for nitric acid than that of copper. If, therefore, caustic potash be 
ured into the impure solution till a blue precipitate ceases to fall, 
it is evident that nothing but oxide of silver remains dissolved. As 
your extensive chemical inquiries must have made you familiar with 
the above subject, and others of a similar nature, it would be most 
acceptable to many of your readers if you would sometimes occupy 
a few pages of the Annals with an examination of the processes by 
which the different metals are obtained ; more particularly as your 
System of Chemistry cannot, from its plan, be quite so diffuse as 
might be desired. M. B.. 
Vox. Vil. N° VI, 2H 
