Elements of chemistry, including the history of the imponderables and the inorganic chemistry of the late Edward Turner / [Edward Turner].

  • Edward Turner
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, including the history of the imponderables and the inorganic chemistry of the late Edward Turner / [Edward Turner]. Source: Wellcome Collection.

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    at 214° in one of glass, owing apparently to its adhering to glass more power¬ fully than to a metal. It is likewise affected by the presence of foreign parti¬ cles : when a few iron filings are thrown into water boiling in a glass vessel, its temperature quickly falls from 214° to 212°, and remains stationary at the latter point. [More recent observations, by Marcet, have showm that when the surface of the vessel, whether of glass or metal, is coated with sulphur, shellac, or other substances of the same kind, boiling takes place at 212°. In this case the tem¬ perature of the w7ater is the same as that of the steam. Without such a coating the boiling point varies In glass vessels from 212°.54 to 215°. 6 In metallic vessels from 212°.27 to 212°.36 (Ann. de Chem.) In this case the temperature of the steam is, of course, less than that of the liquid. M. Magnus in his late able investigations of the same subject, maintains that the various degrees of adhesion of the liquid to the surface of the vessel causes these differences. W hen this adhesion is equal to or exceeds the cohe¬ sion of the particles of the liquid, the particles more remote from the surface being held together only by their cohesion will be converted into steam. The temperature at which this takes place is the true boiling point; but when there is no adhesion of the liquid to the surface, or when this force is less than the cohesion of the liquid, then the boiling will take place in contact with the sur¬ face, because there the restraining force is less, and the boiling temperature will be lower than if the adhesion were equal to or greater than the cohesion. Hence glass, well washed with sulphuric acid so as to promote contact and adhesion, raises the boiling point to 221°. (Ann. de Chem. 1844.)] But the circumstance which has the greatest influence over the boiling point of liquids is variation of pressure. All bodies upon the earth are constantly exposed to considerable pressure; for the atmosphere itself presses with a force equivalent to a weight of 15 pounds on every square inch of surface. Liquids are exposed to this pressure as well as solids, and their tendency to take the form of vapour is very much counteracted by it. In fact, they cannot enter into ebullition at all, till their particles have acquired such elastic force as enables them to overcome the pressure upon their surfaces: that is, till they press against the atmosphere with the same force as the atmosphere against them. Now the atmospheric pressure is variable, and hence it follows that the boiling point of liquids must also vary. The pressure of the atmosphere is equal to a weight of 15 pounds on every square inch of surface, when the barometer stands at 30 inches, and then only does wTater boil at 212°. If the pressure be less, that is, if the barometer fall below 30 inches, then the boiling point of water and every other liquid will be lower than usilal; or if the barometer rise above 30 inches, the temperature of ebullition will be proportionally increased. On this account water boils at a lower temperature on the top of a hill than in the valley beneath it; for as the column of air diminishes in length as we ascend, its pressure must likewise suffer a proportional diminution. The ratio between the depression of the boil¬ ing point and the diminution of the atmospheric pressure is so exact, that it has been proposed as a method for determining the heights of mountains. Accord-
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