PNEUMATU: APliARATUS. 3'79 



and, together with the plate, is removed in the same manper Impiovedap^ 

 as it was applied. The mouth of the jar is turned upwards, t^'a,f,Jf2 ^^'^1 tj^g 

 the plate removed, and the gas submitted to examination : guists. 

 pr, with mouth downwards, the small jar is placed on the 

 shelf of the pneumatic trough, as the experiment may re- 

 quire. This detail appears tedious, but the practice is very 

 easy. Jn this process there is, however, some danger of dis- 

 turbing the lower plate, by lifting it from the mouth pf 

 the magazine jar, and so vitiating the gas by the introduc- 

 tion of common air. To prevent this inconvenience, it is 

 necessary to secure the two perforated plates tp the mouth of 

 the jar, and to each other, allowing the upper plate, at tho 

 same time, to slide freely over the other. For this. purpose, 

 it is necessary to fix the plates, and the magazine jar, in a 

 frame; which renders the use of them very convenient, 

 and not liable to accidental disturbance. 



The two plates (^</ and bj, as in Fig. i„ are fixed in the 

 upper part of the frame .• f^J is fastened f/'J slides easily 

 over it. The jar CJJ is pressed up against the plate r<J by 

 a moveable bottom fkj, tightened by wedges or screws. 

 The jar may be filled with water before it is fixed in the 

 frame, and inverted in the trough ; or the air may be gen- 

 erated in the jar, without the frame, and then, the frame 

 being inverted; and the plates «unk in the water, the jar may 

 be slipped into its place, and fixed there, which is the better 

 way. The frame and jar are then set upright, and the 

 gas may be transferred as before, without danger or loss, or 

 mixture. 



By means of this apparatus, jars of any size may be used as 

 tjiagazines, y/ith the inconvenience of being obliged to in- 

 vert them in large troughs. 



This apparatus, also, on a smaller scale, may be used iu 

 operating with those gasses which can only be confined over 

 mercury. The joints of the transferring plates retain very 

 securely any quantity of mercury, provided the height of the 

 jar is inconsiderable, not more than three, or four inches, for 

 reasons well known to experimental philosophers, Ar^d 

 small jars, with ground mouths, hold mercury very well, 

 when standing, without agitation, with their mouths down- 

 wards, on ground plates of glass. The caretul operator will, 



however. 



