ELASTICITY OF AIR. 



49 



In using this instrument, it is always desirable and frequently necessary to 

 ascertain the degree of rarefaction which has been accomplished within the 

 receiver. This is indicated with great precision, by an apparatus called a 

 barometric-gauge, represented at H, G. This consists of a glass tube H, G, 

 the upper end, H, of which has free communication with the receiver, or rather 

 with the tube T, at some point above the stop-cock C. The tube H, G, is 

 more than thirty inches in length, and its lower extremity is plunged into a 

 small cistern of mercury. As the rarefaction proceeds in the receiver, the 

 elastic force of the air pressing upon the mercury in the tube H, G, is dimin- 

 ished, and immediately becomes less than the pressure of the external atmo- 

 sphere on the surface of the mercury in the cistern M ; consequently this ex- 

 ternal pressure prevails, and forces mercury up to a certain height in the tube 

 H, G. As the rarefaction of the air in the receiver increases, its elastic force 

 being diminished, the atmospheric pressure will prevail with increased effect, 

 and will cause the column sustained in the tube to rise. The weight of this 

 column, combined with the elastic pressure of the air remaining in the re- 

 ceiver, is equal to the atmospheric pressure, because they are balanced by it, 

 and it is therefore apparent that the elastic pressure of the air in the receiver 

 must be equal to the excess of the atmospheric pressure above the weight of 

 the mercurial column in the tuoe. Let us suppose that the common barometer 

 stands at thirty inches, and that the column in the gauge measures twenty-seven 

 inches, the difference between these, namely, three inches of mercury, will 

 express the elastic force of the rarefied air in the receiver, for the column of 

 thirty inches in the barometer measures the atmospheric pressure, and the 

 column of twenty-seven inches in the gauge must be added to the pressure of 

 the rarefied air, in order to obtain the force which balances this pressure ; 

 therefore the force of the rarefied air must be equivalent -to the pressure of 

 three inches, by which the barometric column exceeds the mercurial column 

 suspended in the gauge. 



In small pumps, which are used on the table, gauges of this form are re- 

 jected in consequence of their inconvenient dimensions. An instrument called 

 a siphon-gauge is then used, the principle of which is easily understood. A 



Fig. 5. 



small glass tube, of eight or ten inches in length, is bent into the form A, B, 

 C, D, represented in fig. 5. The extremity A, is closed, and the extremity D, 

 opened, and furnished with a screw, by which it may be attached to a tube 

 connected with the tube T, fig. 4, above the stop-cock C. Pure mercury is 

 poured into the tube A, B, C, D, fig. 5, until the leg A, B, is completely filled, and 

 the mercury rises to S, about half an inch above the inflection B. The pres- 

 sure of the atmosphere communicating freely with the surface S, through D, 

 I C, will maintain the mercury in the space S, B, A, and will prevent the sur- 

 J face S, from rising toward C, by the pressure of the column B, A. When D 



( VOL. II. 



l-S^>^N^^VX>^/-v^-s., 



