Elements of chemistry : including the recent discoveries and doctrines of the science / by Edward Turner.

  • Edward Turner
Date:
1835
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry : including the recent discoveries and doctrines of the science / by Edward Turner. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    Fahr. Cent. Reaum. Fahr. Cent. Reaum. Fahr. Cent. Reanm. 212 100 80 113 45 36 14 -10 - 8 203 95 76 104 40 32 5 -15 -12 194 90 72 95 35 28 - 4 -20 -16 185 85 68 86 30 24 -13 -25 -20 176 80 64 77 25 20 -22 -30 -24 167 75 60 68 20 16 -31 -35 -28 158 70 56 59 15 12 -40 -40 -32 149 65 52 50 10 8 140 6i 48 41 5 4 131 55 44 32 0 0 122 50 40 23 -5 -4 The mercurial thermometer may be made to indicate temperatures which exceed 212°, or fall below zero, by continuing- the degrees above and below those points. But as mercury freezes at 39 degrees below zero, it cannot indicate temperatures below that point; and indeed the only liquid which has been used for such purposes is alcohol. Our means of estimating high degrees of heat are as yet very unsatisfactory. Mercury is preferable to any other liquid; but even its indications cannot be altogether relied on. For, in the first place, its expansion for equal increments of heat is greater at high than at low temperatures; and, secondly, glass expands at tempera- tures beyond 212° in a more rapid ratio than mercury, and consequently, from the proportionally greater capacity of the bulb, the apparent expansion of the metal is considerably less than its actual dilatation. Thus Dulong and Petit observed, that when the air thermometer is at 572°, the common mercurial thermometer stands at 586°; but when corrected for the error caused by the glass, it indicates a temperature of 597-5°. No liquid can be employed for temperatures which exceed 662°, since all of them are then either dissipated in vapour, or decomposed. M. Bellain has observed that mercurial thermometers slowly change their point of zero, which uniformly becomes higher than at the time of gradua- tion. This phenomenon appears owing to a diminished capacity of the bulb due to the atmosphere continually pressing on its exterior, while a vacuum exists in the interior of the tube ; for it has not been noticed either in mercurial thermometers which are unsealed, or in thermometers made •with alcohol. The principal contraction ensues soon after the tube is sealed, and hence some months should be permitted to elapse between the sealing and graduation of a thermometer. (An. de Ch. et de Ph. xxi. 330.) The instruments for measuring intense degrees of heat are called py- rometers, and must be formed either of solid or gaseous substances. The former alone have been hitherto employed, though the latter, from the greater uniformity with which they expand, arc better calculated for the purpose. The action of most pyrometers depends on the elongation of a metallic bar by heat; and the difficulty in their construction consists in find- ing an infusible metal of uniform expansibility, and in measuring the de- gree of expansion with exactness. This subject has for some time occupied the attention of Professor Daniel], who has at last succeeded in forming a pyrometer which, with a little practice, may be used with facility, and ap- pears susceptible of very great precision. (Phil. Trans. 1830 and 31.) This instrument consists of two parts, the Register and Scale, the former designed for exposure to the heat to be estimated, and the latter for measur- ing the exact amount of expansion. The first consists of a bar of black- lead earthenware, in which is drilled a hole 3-10ths of an inch in diameter, and 7£ inches deep. Into this hole a cylindrical bar of platinum or soft iron, of nearly the same diameter, and 6^ inches long, is introduced, so as to rest against the solid end of the hole; and upon the outer or free end of the metallic bar rests a cylindrical piece of porcelain, called the index, 1£ inches long, and is kept firmly in its place by a strap of platinum and a
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