A hand-book of materia medica and therapeutics : with twenty-nine illustrations : being a portion of An analytical compendium of the various branches of medicine / by John Neill and Francis Gurney Smith.

  • Neill, John, 1819-1880.
Date:
1852
Catalogue details

Licence: Public Domain Mark

Credit: A hand-book of materia medica and therapeutics : with twenty-nine illustrations : being a portion of An analytical compendium of the various branches of medicine / by John Neill and Francis Gurney Smith. 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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    moist membranes, which considerably modifies their respective diffu- sibility, on account of their different solubility in water. Under the present head we may most conveniently notice the subject of the physical constitution of the atmosphere and of other gases, and the method of ascertaining specific gravity. Physical Condition of the Atmosphere.—The proof that the atmosphere lias weight is afforded in various ways, although we are not ordinarily sensible of it, as it surrounds us on every side, and is thus maintained in equilibrium. By weighing a large glass receiver before and after its contained air has been exhausted, we shall perceive an obvious difference in the weight. By placing the hand over the mouth of a small receiver, and exhausting the air from the interior, a painful pressure will be experienced from the superincumbent atmosphere. If a bladder be tied over an open- mouthed receiver, and the air be exhausted, the pressure will be sufficient to burst the bladder with a loud report. A similar result follows if a very thin glass receiver be exposed to the exhausting operation of the air-pump. All the above experiments prove that the atmosphere has weight; the amount of this weight is easily shown by the Torricellian experiment, which consists in filling a glass tube, closed at one end, forty or fifty inches long, with mercury, and plunging the open end in a vessel of the same liquid; the mercury will descend to about the level of thirty inches. It is kept at this height by the atmospheric pressure on the mercury in the basin. Now it is ascertained that a column of mercury thirty inches high and one inch square weighs about fifteen pounds; hence the inference that the atmosphere presses upon every square inch of the earth's surface with a weight equal to fifteen pounds. If water had been used in the above experiment, it would have risen to the height of thirty-four feet. The barometer is but a modification of Torricelli's tube. It con- sists of a glass tube, a little over thirty inches long, closed at one extremity; it is filled with mercury, and then inverted so as to place the open end in a small cup of mercury; the pressure of the atmosphere upon the surface of the latter sustains the mercury in the column, as before mentioned. The use of the barometer as a weather-glass dejiends upon the fact that the atmospheric pressure varies at the same place; when it is greatest, the barometer will of course stand highest, and will indicate fair weather, and vice versa. As already remarked, gases are far more elastic than either solids or liquids. The elasticity of the air, and of gases generally, de- pends upon the degree of pressure to which they are subjected. By the law of Mariotte, their density and elastic force are directly as the pressure, and inversely as the volume. Thus, one hundred