“Dp | 
-manufactured in this country. For these, 
Dr. Brewsrer has succeeded in obtain- 
ing a substitute, in a delicate fibre 
which enables the observer to remove 
the error of inflection, while it possesses 
the requisite properties of opacity and 
_elasticity, This fibre. 1s made of glass, 
_which is so exceedingly elastic that it 
may be drawn to any degree of fineness, 
and can always be procured and _pre- 
pared with facility. This vitreous fibre, 
when drawn from a hollow glass. tube, 
will also be of a tubular structure, and 
its iterior diameter may always be 
regulated hy that of the original tube. 
When the fibr®.is formed: and stretched 
across the diaphragm of the eye-piece of 
a telescope, it will appear perfectly 
opaque, with a delicate line of light ex- 
tending along its axis. As this central 
transparency arises from the, transmis- 
sion of the incident Jight through the 
axis. of the hoilow tube, and this tube 
can he made.of any calibre, the dia- 
meter of the luminous streak can be 
either increased or diminished. In a 
micrometer fitted up in this w ay by Dr. 
Brewster, the glass fibres are about 
gao6 Of an inch in diameter; aud the 
irmge of light is dighuely visible, though 
it does not exceed sage of an inch. Tn 
using these fibres tur measuring _ the 
angle subtended by two iuminous points, 
the fibres may be separated, as hitherto 
done, till the luminous points are in 
contact with the interior surfaces; but, 
in order to avoid the error arising from 
inflection, it is proposed to separate the 
fibres, till the rays of light issuing from 
the luminous points dart through the 
transparent axis of the fibres. The rays 
‘thus transmitted evidently suffer no in- 
flection, in passing through the fibre to 
the eye; and, besides this advantage, 
the observer has the benefit of a deli- 
‘cate line, about one-third of the. dia- 
meier of the fibre rtself. 
Mr. Josep Hume. has discovered a 
new method of detecting arsenic. The 
test which he proposes as a substitute 
for those hitherto used, appears to be 
more efficacious, inasmuch as it pro- 
duces a more copious precipitate from 
a given quantity of that substance. It 
is composed im the following manner :— 
Let one grain of white oxide of arsenic, 
and the same quanti ty of carbonate, of 
soda, be dissolved by boiling in ten or’ 
twelve ounces of distilled water, which 
ought to be done in a glass vessel ; to 
this, Ict.a.small quantity of the nitrate 
ef silver be added, and a y bright yellow 
Literary and Philosophical Intelligence. 
copper, 
[July 1 ; 
precipitate will instantly appear. This 
Is amore decisive test than sulphate of 
‘which forms Scheele’s-yreen, 
(arseniate of copper) and though «the 
process answers very well with potash or 
_jime-water, yet. Mr. Hume ‘is. inclined - 
to prefer the common sub-carbonate of 
soda. 
A correspondent of the Philoso#lical 
Magazine, taking into consideration the 
present imperfect mode. of finding the 
rates of time-keepers, suggests, the 
establishment of a public observatory 
for trying time-keepers and ‘keeping 
their rates, to which every maker, if he 
thought proper, might have access at 
stated hours, and where he might be al- 
lowed always to keep a certain. limited 
number of. pieces. Here he might try 
the effect of improvements and gain ex- 
perience; then alter and try again ‘till 
he succeeded to his mind; an advan- 
tage which he could not, perhaps, enjoy 
in his own house, for .want-of Instru- 
ments of sufficient accuracy and leisure 
to make the necessary.computations. A 
book containing the rate of each time-~ 
keeper might be kept always ready for the 
use of the owner, and, if he thought pro-’ 
per, for the inspection: of the public, by. 
which he would be enabled to fix a price 
on the machine, proportioned to the ex- 
cellence of its going. From this place 
captains of ships and~éthers might al- 
ways be furnished with timekeepers, 
suitable to the price they could afford, or 
adapted, with respect to accuracy. of 
going, to the purposes:for which they 
might be required. ‘The writer expresses 
his surprize that, considermg the many - 
evident advantages of such an. institu- 
tion, the watch-makers have not already 
established one at-their own expense. 
That valuable plant, smyrna madder, 
has lately been introduced, into this 
country by Mr. Spencer SmitH, who 
furpished the Society of Arts with sone 
seed; from which Mr. Salisbury, of the 
Botanic Garden, Sloane-street, has raised 
plants that have grown in the most pro- 
mising mannery He expects to obtain 
seed from them, and theré is every 
reason to hope that this useful ks boa 
will become naturalized in our soil. 
When the Freneh siezed Liege, ‘the 
gentlemen belonging to the seminary 
of that place were obliged tomakeapreci- - 
pitate retreat, abandoning a large esta-- 
blishment, together with a. valuable’ lis ” 
brary anda fine colléction of mathe= 
matical - instruments. ‘Having since 
found an asylum in this. country, they 
| <9 have 
