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

  • Turner, Edward, 1798-1837.
Date:
1829
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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    above and below those points. But as mercury freezes at 40 degrees below zero, it cannot indicate temperatures below that point; and indeed the only liquid which can be used for such purposes is alcohol. Our means of estimating high degrees of heat are as yet very unsatis- factory. Mercury is preferable to any other liquid ; but even its indi- cations cannot be altogether relied on, because glass expands at temperatures beyond 212' F. in a more rapid ratio than mercury ;and, therefore, from the proportionally greater capacity of the bulb, the apparent expansion of the metal is considerably less than its actual dilatation. No liquid can be employed for temperatures which ex- ceed 6003 F. since all of them are then either dissipated in vapour, or decomposed. The instruments for measuring intense degrees of heat are called pyrometers, and must be formed either of solid or gaseous substances. The former alone have been hitherto employed, though it is probable that the latter, from the greater uniformity with which they expand, are better calculated for the purpose. The pyrometer invented by Mr Wedgewood is best known. It is founded on the property which clay possesses of contracting, when strongly heated, without returning to its former dimensions as it cools. The earth alumina, whether pre- pared by common chemical processes, or found more or less pure in the earth as clay, is always in a state of chemical combination with water. On heating it to redness, a part of the water is expelled ; but some remains, which requires a very strong heat before it is dissipa- ted ; and in proportion as these last portions escape, the earth con- tracts. The contraction even continues after every trace of water has been removed, owing to a partial vitrification taking place, which tends to bring the particles of the clay into nearer proximity. The in- tensity of the heat may, therefore, in some measure be estimated by the degree of contraction which it has occasioned. The apparatus consists of a metallic groove, 24 inches long, the sides of which converge, being half an inch wide above, and 3-tenths below. The clay, well washed, is made up into little cubes that fit the commencement of the groove, after having been heated to red- ness ; and their subsequent contraction by heat is determined by allow- ing them to slide from the top of the groove downwards, till they arrive at a part of it through which they cannot pass. Mr Wedgewood divides the whole length of the groove into 240 degrees, each of which he supposes equal to 130° F. The zero of his scale corresponds to 1077 of Fahrenheit. Wedgewood's pyiometer is little employed at present, because its indications cannot be relied on. Every observation requires a sepa- rate piece of clay, and the observer is never sure that the contraction of the second cube, from the same heat, will be exactly similar to the first; especially as it is difficult to procure specimens of the earth, whose composition is in every respect the same. Other pyrometers have been proposed, which act on the usual prin- ciple of dilatation. They consist of a metallic bar, the elongation of which, from heat, is rendered sensible by an index being attached to one end, while the other is fixed. The experiments of Lavoisier and Laplace on the expansion of solids were made with such an apparatus, and Mr Daniell has lately described a similar one in the 11th volume of the London Journal of Science. These instruments are in general too complicated for common use ; and, moreover, scientific men have