594 



THERMOMETER. 



bag. The column of mercury should not < 

 half an inch in length. By gently inclining the 

 tube, and tapping it with the finger, we bring the 

 mercury to about a couple of inches from the end 

 where we mean to make the bulb, and, with a file 

 or diamond, mark there the initial line of the scale. 

 The slip of ivory, brass or paper, destined to receive 

 the graduations, being laid on a table, we apply the 

 tube to it so that the bottom of the column of mer- 

 cury coincides with its lower edge. With a fine 

 point, we then mark on the scale the other extre- 

 mity of the mercurial column. Inclining the tube 

 gently, and tapping it, we cause the liquid to flow 

 along till its lower end is placed where the upper 

 previously stood. We apply the tube to the scale, 

 taking care to make its initial line correspond to the 

 edge, as before. A new point for measuring equal 

 capacity is now obtained. We thus proceed till 

 the requisite length be graduated, and we then 

 weigh the mercury with minute precision. The 

 bulb is next formed at the enameller's blow-pipe, in 

 the usual way. One of a cylindrical 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 descend 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 quick- 

 silver will descend from the paper funnel, and com- 

 pletely fill the bulb and stem. Should any portion 

 of air appear, the process ofheating 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 im- 

 mersed 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 expelled. 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 mer- 

 curial column. This point must be noted with a 

 scratch on the glass, or else by a mark on the pre- 

 pared scale. We then weigh the whole. We have 

 now sufficient data for completing the graduation of 

 the instrument from one fixed point ; and, in hot 

 climates, and other situations, where ice, for exam- 

 ple, cannot be conveniently procured, this facility 

 of forming an exact thermometer is important. 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 to 100 cent., cor- 

 responds to a mercurial expansion in glass of one 

 sixty-third, we ran 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 initial measuring column were 0-6 of a grain, 

 then ten of these spaces would comprehend the range 

 betwe.cn 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 tlc- 

 grees of Fahrenheit ; or into ten equal parts, which 

 would constitute centigrade degrees ; or into eight, 

 which would form Reaumur's degrees. When \\r 

 have ice and boiling water at hand, however, we 

 may dispense with the weighing processes. By 

 plunging the instrument into melting ice, and tin 'ii 

 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 sufficient guide. Thermome- 

 ters are used for two different purposes, each of 

 which requires peculiar adaptation. Those em- 

 ployed in meteorology, or for indicating atmos- 

 pherical temperature, are wholly plunged in tho 

 fluid ; and hence the stem and the bulb are equally 

 affected by the calorific energy. But wlu-n tin: 

 chemist wishes to ascertain the temperature of cor- 

 rosive 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., un- 

 acted on, has, at 212, escaped from its influence. 

 Hence a meteorological and a chemical thermome- 

 ter 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 im- 

 mersed 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 ex- 

 periment should have its boiling point determined 

 by immersion only of the bulb, and the naked por- 

 tion 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 seal- 

 ing 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 rapidly 

 from one end of the tube to the other when it is 

 inverted with a jerk. An ivory scale is the hand- 

 somest, 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 direc- 

 tion to the thermometer stem. It is soldered to 

 it above by the lamp, and hooked to it below by a 

 ring of glass. 



Like most other important discoveries, that of 

 the thermometer has had several claimants, the 

 respective merits of whom it seems nearly impos- 

 sible at the present day definitively to determine. 

 Though by some Galileo has been considered as the 

 true inventor, and by others, Father Paul Sarpi 

 the Venetian, this honour chiefly rests between 

 Sanctorius, an Italian physician, and Drebbel, a 

 Dutchman, both men of ingenuity and original 

 minds. Many moderns support each of these 

 claims, but Sanctorius (or Santorio,) has on the 

 whole most in his favour, since, as Martine justly 

 observes, no one but he demanded the honour of 

 an inventor during his lifetime. His Commentaries 

 on Aviccnna, published in 1626, are very interest- 



