A manual of elementary chemistry : theoretical and practical / by George Fownes.

  • Fownes, George, 1815-1849.
Date:
1868
Catalogue details

Licence: Public Domain Mark

Credit: A manual of elementary chemistry : theoretical and practical / by George Fownes. Source: Wellcome Collection.

    43/608 (page 51)
    Fig. 18. the volume of air is reduced to a third, it will be found that the column mea- sures GO inches, and so in like proportion as fur as the experiment is carried. The above instrument is butter adapted for illustra- tion <>f the principle than for furnishing rigorous proof of the law; this has, however, been done. MM. Arago and Dulong published, in the year 1830, an account of certain experiments made by them in Paris, in which the law in question had been verified to the extent of '21 atmospheres. Anil with rarefied air, of whatever dearer of rarefaction, the law has been found true. All gases are alike subject to this law, and all vapors of volatile liquids, when remote from their points of liquefaction.1 It is a matter of the greatest importance iu practical chemistry, since it gives the means of making corrections for pressure, or determining by cal- culation the change of volume which a gas would suffer by any given change of external pressure. Let it be required, for example, to solve the following problem:—We have 100 cubic inches of gas in a gradu- ated jar, the barometer standing at 29 inches: how many cubic inches will it occupy when the column rises to 30 inches ?—Now the volume must be inversely as the pres- sure ; consequently a change of pressure in the propor- tion of 29 to 30 must be accompanied by a change of volume in the proportion of 30 to 29; 30 cubic inches of gas contracting to 29 cubic inches under the conditions imagined. Hence the answer:— 30 : 29 = 100 : 96-67 cubic inches. The reverse of the operation will be obvious. The prac- tical pupil will do well to familiarize himself with these simple calculations of correction for pressure. l-'rum what has been said respecting the easy com- ])i<.---ibility of gases, it will be at once seen that the atmosphere cannot have the same density, and cannot exert equal pressures at different elevations above the sea-level, but that, on the contrary, these must diminish with the altitude, and very rapidly. The lower strata of air have to bear the weight of those above them; they become, in consequence, denser and more compressed than the upper portions. The following table, which is taken from Prof. Graham's work, shows in a very simple manner the rule followed in this respect:— Heteht above the Height of barometer, eva. in miles. Volume of air. in inches. 0 1 30 2-705 2 15 5-41 4 ... .. 7-5 8-115 8 3-75 lu-vj 16 1-875 32 .. 0-9375 1023 64 1.....'..'.'...!. 0-46875 The numbers in the first column form an arithmetical series, by the constant addition of - 7«>>: tho-e in the second column an increasing geometrical series, each being double its predecessor; and those in the third, a decreasing geo- metrical series, in which each number is the half of that standing above it 1 When near tho H^U. ivin- point the law no longer holds; the volume diminishes mart r-ijiidly than th« theuq indicates, a smaller amount of pressure being then sufficient.