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

  • George Fownes
Date:
1868
Catalogue details

Licence: Public Domain Mark

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

    42/608 (page 50)
    Fig. 17. surface of the liquid. Now, if the density of this liquid wore mown, an(| the height and area of the column measured, means Avould be furnished for exaetly estimating the amount of pressure exerted by the atmosphere. Such an instrument is the barometer: a straight glass tube i> taken, about 30 inehes in length, and sealed by the blowpipe flame at one extremity ; it is then filled with clean, dry mercury, care being taken to displace all air-bubbles, the open end stopped with a finger, and the tube inverted in a basin of mercury. On removing the finger, the fluid sinks away from the top of the tube, until it stands at the height of about 30 inches above the level of that in the basin. Here it remains supported by, and balancing the atmospheric pressure, the space above the mer- cury in the tube being of necessity empty. The pressure of the atmosphere is thus seen to be capable of sustaining a column of mercury 30 inches in height, or there- abouts: now such a column, having an area of one inch, weighs between 14 and 15 pounds, consequently such must be the amount of the pressure exerted upon every square inch of the surface of the earth, and of the objects situated thereon, at least near the level of the sea. This enormous force is borne without inconvenience by the animal frame, by reason of its perfect uni- formity in every direction, and it may be doubled, or even tripled without injury. A barometer may be constructed with other liquids besides mercury; but, as the height of the column must always bear an inverse proportion to the density of the liquid, the length of tube required will be often considerable; in the case of water it will exceed 33 feet. It is seldom that any other liquid than mer- cury is employed in the construction of this instrument. The Royal Society of London possessed a water-barometer at their apartments at Somerset House. Its construction was attended with great difficulties, and it was found impossible to keep it in repair. It will now be necessary to consider a most important law which connects the volume occupied by a gas with the pressure made upon it, and which is thus expressed:— The volume of a gas is inversely as the pressure; the density and elastic force are directly as the pressure, and inversely as the volume. For instance, 100 cubic inches of gas under a pressure of 30 inches of met eury would expand to 200 cubic inches were the pressure reduced to one-hall and shrink, on the contrary, to 60 cubic inches if the original pressure wer- doubled. The change of density must necessarily be in the inverse propo? tion to that of the volume, and the elastic force follows the same rule. This, which is usually called the law of Mariotte, though really discovere by Boyle (1661), is easily demonstrable by direct experiment. A glass tnlx; about 7 feet in length, is closed at one end, and bent into the form represents in fig. 18, the open limb of the syphon being the longer. It is next attache- to a board furnished with a movable scale of inches, and enough mercury i introduced to fill the bend, the level being evenly adjusted, and marked upoi the board. Mercury is now poured into the tube until it is found that th- ••1 air ha* heeii reduced to one-half of its former volume; and on apply ing the scale, it will be found that the level of the mercury in the open par very nearly 30 inches above that in the closed portion. Tlu pressure of an additional atmosphere has consequently reduced the bulk of the contained ail to one-half. If the experiment be still continued until