Prof. Hare's improved Eudiometrical Apparatus. 131 



munication with the air-pump may be established. The other 

 pipe is used when different kinds of gas are to be successively 

 introduced, or when a portion of residual gas is to be drawn 

 out for examination. T, T are rods, by means of which the 

 platina wire communicates with the poles of a calorimotor, so 

 as to be ignited, by being the medium of discharge, as often 

 as the surfaces are excited by the acid. The calorimotor* 

 employed is so constructed, as that the revolution of a wheel 

 through a quarter of a circle, is sufficient to raise the vessel 

 holding the acid until the galvanic plates are surrounded by it. 

 At m is a wooden dish holding mercury for the gauge-tube. 



It is well known to those who are familiar with pneumatics, 

 that if a receiver communicate simultaneously with an air-pump 

 and a barometer-gauge, the extent of the exhaustion will be 

 indicated by the height of the mercury in the gauge-tube ; so 

 that if there be a scale of equal parts associated with the tube, 

 the quantity of air taken from the receiver at any stage of the 

 exhaustion will be to the quantity held by it when full, as the 

 number opposite the upper extremity of the mercurial column, 

 when the observation is made, to that to which it would ex- 

 tend if the receiver were thoroughly exhausted. 



Hence, if on exhausting the vessel thoroughly the mercury 

 rise 450 degrees, on admitting a gas freely, 450 parts of the 

 gas would replace the air withdrawn ; but if the entrance of 

 the gas be restricted, so that a mercurial column is still sus- 

 tained in the tube, the quantity of gas which has entered will 

 be as much less than 450, as the mercury is above 0. Thus 

 for instance, let the mercurial column sustained extend to 1 50 

 on the scale, 300 parts of gas will have entered, and if by ex- 

 plosion or any other means any number of parts of the gas, 

 thus introduced, be condensed, the mercury must rise that 

 number of degrees in the gauge f. 



* See Phil. Mag. vol. liv. p. 209. 



f That portion of the bore of the tube which is not occupied by mer- 

 cury, adds to the capacity which influences the gauge, and the portion of 

 the gauge which is emptied of mercury varies in extent; but as the air 

 which remains in the gauge is not subjected to the explosion, the extent 

 of the condensation is uninfluenced by it. 



A slight error may arise from the sinking of the mercury in the dish, as 

 the quantity in this receptacle lessens by its rise in the tube; and vice 

 versa when subsidence ensues. This movement will be to the movement 

 of the mercurial column in the tube, as the square of its diameter to the 

 square of the diameter of the mercurial stratum in the dish ; and the diame- 

 ters of these being respectively as 20 to 1, it would be 1-400 of the whole 

 height of the scale : this difference may be allowed for, or may be remedied, 

 by raising or lowering the dish, by an appropriate screw, or employing a 

 dish of a superficies so large, and a gauge-tube with a bore so small, as to 

 render the effect of the rise, or subsidence of the mercury in the gauge, in- 

 significant. 



S2 The 



