Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
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Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

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    in such a vessel inverted above water, it makes its escape from the vessel, and passes through the water. Thermometers are instruments for measur- ing degrees of heat by the expansion of fluids in narrow tubes. Mercury is generally used, of which 100,000 parts at the freezing point of water become 101,835* parts at the boiling point, and on Fahrenheit’s scale these parts are divided into 180 degrees. Solids, by a certain increase of heat become fluids, and fluids gases, or elastic fluids. Thus ice is converted by heat into water, and by still more heat it becomes steam: and heat disappears, or, as it is called, is rendered latent during the conversion of solids into fluids, or fluids into gases, and reappears or becomes sensible when gases become fluids, or fluids solids; hence cold is produced during evaporation, and heat during the condensation of steam. There are a few exceptions to the law of expansion of bodies by heat, which seem to depend either upon some change in their chemical con- stitution, or on their becoming crystallized. Clay contracts by heat, which seems to be owing to its giving off water. Cast iron and anti- mony, when melted, crystallize in cooling and expand. Ice is much lighter than water. Water expands a little even before it freezes, and it is of the greatest density at about 41° or 42°, [at 39° according toHallstrom.—S] the freezing point being 32°; and this circumstance is of considerable im- portance in the general economy of nature. The influence of the changes of seasons and of the position of the sun on the phenomena of vegetation, demonstrates the effects of heat on the functions of plants. The matter absorbed from the soil must be in a fluid state to pass into their roots, and when the surface is frozen they can derive no nourishment from it. The activity of chemical changes likewise is increased by a certain increase of temperature, and even the rapidity of the ascent of fluids by capillary attraction. This last fact is easily shown by placing in each of two wine glasses a similar hollow stalk of grass, so bent as to discharge any fluid in the glasses slowly by capillary attraction; if hot water be in one glass, and cold water in the other, the hot water will be discharged much more rapidly than the cold water. The fermentation and decomposition of animal and vegetable substances require a certain degree of heat, which is consequently necessary for the preparation of the food of plants ; and as evaporation is more rapid in proportion as the temperature is higher, the superfluous parts of the sap are most readily carried off at the time its ascent is quickest. Two opinions are current respecting the nature of heat. By some philosophers it is conceived to be a peculiar subtile fluid, of which the particles repel each other, but have a strong attraction for the particles of other matter. By others it is considered as a motion or vibration of the particles of matter, which is supposed to differ in velocity in different cases, and thus to produce the different degrees of temperature, f What- ever decision be ultimately made respecting these opinions, it is certain that there is matter moving in the space between us and the heavenly bodies capable of communicating heat; the motions of which are recti- lineal: thus the solar rays produce heat in acting on the surface of the * 100,000 parts of Mercury, at 32° become 101,8018 at 212° or increase j~ of their volume, according to Dulong and Petit. t Those who hold the vibracular hypothesis rather maintain that heat is the result of waves or vibrations of an extremely attenuated ether, that pervades all spaoe and matter. c