rn 
ceed till the requisite length be graduated, and 
we then weigh the mercury with minute preci- 
sion. The bulb is next formed at the enameller’s 
blow-pipe, in the usual way. One of a cylindri- 
cal or conical shape is preferable to a sphere, both 
for strength and sensibility. We now ascertain 
and note down its weight. A tubular coil of 
paper is to be tied to the mouth of the tube, 
rising in a funnel-form an inch or two above it. 
Into this we pour recently boiled mercury, and, 
applying the gentle heat of a lamp to the bulb, 
we expel a portion of the air. On allowing the 
bulb to cool, a portion of the mercury will de- 
scend into it, corresponding to the quantity of 
air previously expelled. The bulb is now to be 
heated over the lamp till the included mercury 
boil briskly for some time. On removing it, the 
quicksilver will descend from the paper funnel, 
and completely fill the bulb and stem. Should 
any portion of air appear, the process of heating 
or boiling must be repeated, with the precaution 
of keeping a column of superincumbent mercury 
in the paper funnel. When the temperature of 
the bulb has sunk to nearly that of boiling water, 
it may be immersed in ice-water. The funnel 
and its mercury are then to be removed, and the 
bulb is to be plunged into boiling water. About 
one sixty-third of the mercury will now be ex- 
pelled. On cooling the instrument again in 
melting ice, the zero point of the centigrade 
scale, corresponding to 32° of Fahrenheit, will be 
indicated by the top of the mercurial column. 
This point must be noted with a scratch on the 
glass, or else by a mark on the prepared scale. 
We then weigh the whole. We have now suffi- 
cient data for completing the graduation of the 
instrument from one fixed point; and, in hot 
climates, and other situations, where ice, for 
example, cannot be conveniently procured, this 
facility of forming an exact thermometer is im- 
portant. We know the weight of the whole 
included mercury, and that of each gradus of the 
stem. And, as from 32° to 212° Fahr., or from 
0° to 100° cent., corresponds to a mercurial ex- 
pansion in glass of one sixty-third, we can easily 
compute how many of our graduating spaces are 
contained in the range of temperature between 
freezing and boiling water. Thus supposing the 
mercurial contents to be 378 grains, one sixty- 
third of that quantity, or six grains, correspond 
to 180 of Fahrenheit’s degrees. Now, if the ini- 
tial measuring column were 0°6 of a grain, then 
ten of these spaces would comprehend the range 
between freezing and boiling water. Hence, if 
we know the boiling point, we can set off the 
freezing point; or, from the temperature of the 
living body, 98° Fahr., we can set off both the 
freezing and boiling points of water. In the 
present case, we must divide each space on our 
prepared scale into eighteen equal parts, which 
would constitute degrees of Fahrenheit; or into 
ten equal parts, which would constitute centi- 
grade degrees; or into eight, which would form | 
THERMOMETER. 
Réaumur’s degrees. When we have ice and 
boiling water at hand, however, we may dis- 
pense with the weighing processes. By plung- 
ing the instrument into melting ice, and then 
into boiling water, we find how many of our 
initial spaces on the stem correspond to that 
interval of temperature, and we subdivide them 
accordingly. If the tube be very unequal, we 
must accommodate even our subdivisions to its 
irregularities, for which purpose the eye is a suf- 
ficient guide. 
Thermometers are used for two different pur- 
poses, each of which requires peculiar adaptation. 
Those employed in meteorology, or for indicating 
atmospherical temperature, are wholly plunged 
in the fluid; and hence the stem and the bulb 
are equally affected by the calorific energy. But 
when the chemist wishes to ascertain the tem- 
perature of corrosive liquids, or bland liquids 
highly heated, he can immerse merely the bulb 
and the naked part of the stem under the scale. 
The portion of the tube corresponding to the 
scale is not influenced by the heat, as in the 
former case; and hence one sixty-third part of 
the mercury, which, at 32° Fahr., was acted on, 
has, at 212°, escaped from its influence. Hence 
a meteorological and a chemical thermometer 
ought to be graduated under the peculiar con- 
ditions in which they are afterwards to be used. 
The former should have its stem surrounded 
with the steam of boiling water, while its bulb 
is immersed an inch or two beneath the surface 
of that liquid, the barometer having at the time 
an altitude of thirty inches. A thermometer for 
chemical experiment should have its boiling 
point determined by immersion only of the bulb, 
and the naked portion of its stem below the 
scale, in boiling water. The water, of course, 
must be pure; and it ought to be contained in a 
metallic vessel. Before sealing up the end of the 
tube, we should draw it into a capillary point, 
and heat the bulb till the mercury occupy the 
whole of the stem. A touch of the blow-pipe 
flame on the capillary glass will instantly close 
it, and exclude the air from re-entering when 
the bulb becomes cool. If this has been skilfully 
executed, the column of mercury will move ra- 
pidly from one end of the tube to the other when 
it is inverted with a jerk. An ivory scale is the 
handsomest, but the most expensive. Those 
used in Paris consist of a narrow slip of paper 
enclosed in a glass tube, which is attached in a 
parallel direction to the thermometer stem. It 
is soldered to it above by the lamp, and hooked 
to it below by a ring of glass. 
Comparative Scales of Thermometers.—A fertile 
cause of error in estimating and comparing the 
statements of temperature, is the very different 
manner in which they are made by scientific 
men of different nations. Wherever the English 
language prevails, the graduation of Fahrenheit 
is generally preferred. By the German authors 
Réaumur is used ; and the French have, within 
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