Chemistry / by George Wilson.

  • George Wilson
Date:
1857
Catalogue details

Licence: Public Domain Mark

Credit: Chemistry / by George Wilson. Source: Wellcome Collection.

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    temperatures; that is to say, an amount of heat which would expand them to a certain extent, if they, at the period of its application, were at the temperature of the air, would expand them to a greater extent if, at the moment of its reception, they were at a higher temperature, such as a red heat. This has not been observed, howevei’, to be the case with gases. THE THERMOMETER. 212. The thermometer is an instrument in which tempera- ture—that is, the intensity of heat—is measured by the amount of expansion it produces on solids, liquids, or gases. Solids expand too little, gases too much, to be practically available for the construction of thermometers. The only one, accord- ingly, to which we shall refer is the liquid thermometer uni- versally in use. 213. The liquids employed to measure heat are two; namely, spirits of wine or alcohol, and quicksilver. The for- mer is employed for instruments intended to measure low temperatures, for which purpose quicksilver is inapplicable, as it freezes, or becomes solid, if too much heat be withdrawn from it; whereas the strongest spirits of wine cannot be con- gealed by any known cold. For ordinary temperatures, how- ever, mercury is preferable to alcohol. 214. That the expansions and contractions of the mercury or spirits of wine may be visible, the liquid is enclosed in a vessel of transparent glass. This consists of a glass tube, occupied throughout its entire length by a very narrow canal, which ends at the one extremity in a hollow ball or pear- shaped chamber. At the end furthest from this chamber or bulb, the tube is left open till the liquid has been introduced by a peculiar process, which will be found described in works treating specially of heat. The opening is then closed by heating the glass till it melts and runs together, forming as it cools a solid plug, shutting’ up the previously open mouth of the tube. The quantity of liquid introduced is generally little more than sufficient to fill the bulb at the lowest tempera- ture which the thermometer is intended to indicate. 215. The instrument, as thus constructed, would only tell that one body was hotter or colder than another, without showing by how much the one exceeded or fell short of the other in temperature. Before it can record this, it requires to be what is called graduated; in other words, to have points marked on it which correspond to certain invariable tempera- tures or intensities of heat. 216. If ice or snow be placed in water, and allowed to melt
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