Encyclopaedia Britannica > Volume 7, ETM-GOA

1
His third
eudiome-
E U D [
vefTel; then let the phial iV, when filled with water,
be put into CED, the infide focket of the tin-veffel.
Fill it then with nitrous air ; and let this quantity be
thrown into the phial ABC, which is to be fixed fome-
what tight to the mouth C of the eudiometer. The
fame phial N is afterwards filled with the air of wdiich
we wilh to try the quality ; and raifing the end of the
inftrument C, it is then put into the mouth E. The
inftrument is then to be placed upright as in the fi¬
gure, by hanging it on the hook fT; and as foon as
this laft air goes up to the r'phial ABC, the phial N is
to be taken off,, that the diminution of the two mixed
airs may be fupplied from the water in the tin-veffel :
the mouth E o( the eudiometer being all this time held
under water. The bent tube gh, having the brafs
ring K fitted to it, is then put to the lower end E o{
the eudiometer. By obferving the furface of the wa¬
ter in the fmall tube, which thus forms a £rue fyphon
with the tube of the inftrument, and by means of the
brafs ring K, the ftationary ftate of diminution in the
mixture may be diftinguilhed; which being afcertained,
the fmall tube ^ A is taken off from the eudiometer, and
the whole inftmment laid down for fome minutes in
the water of the tin-veffel; after which the mouth E
is to be Ihut up with the glafs-ftopple JU; and, rever-
fing the inftrument, it is hanged up by the end E up¬
on the hook IV. By thispofition the whole diminilh-
ed air of the veffel ABC goes up to the top, where its
real bulk is fhown by the fcale facing the infide furface
of the water. This number being dedu&ed from 128,
gives the comparative wholefomenefs of the air already
tried without any farther calculation. “ But this pro-
cefs (fays Mr Magellan) will be ftill eafier, when the
laft diminution of the two kinds of air is only required
in the obfervation; becaufe no ufe will then be made
of the fyphon. In fuch a cafe the inftrument is left
hanging on the hook IV for 48 hours; after which it
is laid down under the water of the trough in an hori¬
zontal pofition for 8 or 12 minutes, in order to ac¬
quire the fame temperature with the water: the mouth
E is then fhut up with the ftopple M; the inftrument
is hung by the end E in a contrary pofition ; and the
laft real bulk of the good mixed .air will then be fhown
by the number of the brafs fcale anfwering to the infide
furface of the water.
IV. The third eudiometer conftru&ed by Mr Ma¬
gellan is reprefented fig. 3. where EN reprefents an
uniformly cylindrical glafs-tube about two or three
feet long,, with a large ball 5 and a glafs ftopple M,
fitted air-tight to the mouth iV, which ought to be
wide and funnel-lhaped, unlefs ^feparate funnel is made
ufe of. KL is a fmall fyphon with a brafs ring X;
Z admail phial,, the contents of which do not exceed
one third of the ball S, or one half of the glafs tube.
JLaftly, the inftrument has a ruler I, divided and
ftamped like the fcale already mentioned, with a glafs
funnel, which is ground to the mouth N of the inftru¬
ment, when this is not funnel-fhaped as above directed.
When this eudiometer is to be made ufe of, it muft be
filled with water, and fet in a vertical pofition, with
21 ] E U D
the mouth N under the furface of the water in a tub Eudloine-
or trough. The phial Z is to be filled with nitrous ter-
air, and thrown into the tube by means of a glafs fun- -y—**
nel, if the mouth of the eudiometer tube be not fuffi-
ciently wide to anfwer the purpofe. The fame phial
Z is then to be filled with the air to be tried; after
which the fyphon KL is to be immediately added to
the mouth N of the eudiometer under the furface of
the water, fome of which is to be poured into it.
The ftationary moment of the greateft diminution of
the two airs is watched by means of the ring X; and,
when that moment arrives, the fyphon KL is to be
taken off; the eudiometer is laid for fome minutes un¬
der water in an horizontal pofition or nearly fo; but
taking care that none of the inclofed air may efcape:
the mouth N is then fhut up with the glafs ftopple M,
and the inftrument is inverted with the mouth N up¬
wards. Laftly, the fpace occupied by the refiduum
of the diminifhed air is meafured by applying to its
fide the divided ruler or fcale, and the refult is eftima-
ted as has been already explained. g
On all thefe eudiometers it is very obvious to remark, Inconveni-
that they are complicated and difficult to be ufed; and enccs uf all-
it is befides no eafy matter to get them made with the
requifite accuracy. Mr Cavallo obferves alfo, that the
conftruftion of all the three is founded on a fuppofi-
tion that the mixture of nitrous and atmofpherical air,
after having continued for fome time to diminifh, in-
creafes again ; but he informs us that this is a
miftake, and that Mr Magellan himfelf owned it to-
be fo. But the worft of all is, that they are by no
means accurate, as appeared from feveral experiments
made by Mr Magellan in Mr Cavallo’s prefence, with-
air taken out of the window of the room where the
experiments were performed. By the firft trial, the.
diminution was 48 parts out of 132.0!the mixture: on.
a fecond trial, the fame elaftic fluids being ftill ufed».
the diminution was 58 parts out of 132 : on a third,
trial, the diminution was again 48 ; and by a fourth
one, it was 51. Nay, Mr Magellan himfelf owned that,
after many experiments with his eudiometers, he never
could obtain any conftant refult, even when the nitrous
and commpn air which.he made ufe of were precifely
of the fame quality.
V. A preferable method of difcovering the purity Fontana’s
of the air by means of an eudiometer is recommended eudiome-
by M. Fontana; of which Mr Cavallo fays,, that its ac-,er*
curacy is fuch as could fcarce be believed by thofe who-
have not had an opportunity of obferving it. The in¬
ftrument is originally nothing more than a divided glafs
tube, though the inventor afterwards aided to it a
complicated apparatus, which, in Cavallo’s opinion*
was altogether ufelefs. The firft fimple eudiometer*
confifted only of a glafs tube, as uniformly cylindrical
as poffible in its cavity, about 18 inches long, and
^ths of an inch in diameter in the infide, hermetically
fealed at one end (a). The outfide of this tube was.-
marked with a diamond, or had circles drawn round it
at the diftance of three inches from one another, be¬
ginning at the clofed end of the tube ; or at fuch di-
ftances
(a) To obferve whether the cavity of a glafs tube is perfectly cylindrical, pour into it at different times -
equal quantities of mercury or water, one upon the other ; obferving each timej by means of a divided .rulera ,
if thofe equal quantities of liquor fill equal lengths of the tube.