1876.] 



Sprengel Air-pump and Vacuum-tap. 



397 



that when A is full the level of the mercury shall be just above the 

 ends of the fall -tubes (A, ^, j) in the fixed reservoir B. 



A sufficient quantity of mercury being poured into B, the pinch-cock 

 (K) is opened, when the mercury flows through the flexible tubing* into 

 A, in the direction shown by the arrows, till the level in each reservoir 

 is the same ; K is then closed and A raised to the stop S'. S" is another 

 stop for the reservoir A, placed halfway up the stand, so that when the 

 exhaustion has proceeded sufficiently to cause the mercury to rise to the 

 necessary height, by the excess of external pressure, much labour may be 

 saved by only raising the reservoir to this middle stop. 



The pinch-cock K being closed and the reservoir A raised, the mer- 

 cury passes the three-way connexion (L) up a small length of flexible 

 tube to the pinch-cock M, thence up the glass tube a, through the air- 

 trap h, and, rising over the point falls into the jet e, where it is 

 divided into three columns, supplying the three fall-tubes (A, ^, ji"), and, 

 having taken out a certain amount of air in its passage down these, is 

 collected in B. 



The exhaustion is carried through the arm /, with which it will be 

 seen are connected, by means of mercury joints and the author's device 

 for a vacuum-tap, the different gauges, &c. considered useful when 

 working with high exhaustions, g is an ordinary barometer-gauge 

 dipping into a separate reservoir C, having the barometer (k) on one side 

 and the measuring-rod {I) on the other; the latter is a glass tube divided 

 into millimetres measured from a point at the bottom, which is always 

 made to touch the surface of the mercury in C before taking an obser- 

 vation of the height of the gauge or barometer. 



m represents a small siphon-gauge, but is only of little value, owing 

 to its small bore and the consequent interference of capillarity ; this, how- 

 ever, could at any time be replaced by one of a larger bore where occa- 

 sion to require its accurate use. 



n is one of Mr. Crookes's radiometers, made on a small scale, consti- 

 tuting a most valuable gauge as to the exhaustion within the pump, as its 

 rate of motion increases almost to the highest exhaustions obtained by the 

 pump alone ; and even up to an absolute vacuum, the radiometer would 

 give under any circumstances valuable indications with regard to the state 

 of exhaustion, as will be seen in future papers by Mr. Crookes. 



The wide tube o is the sulphuric or anhydrous phosphoric acid 

 reservoir. 



Connected to the exhaust arm by the spiral P is McLeod's beautiful 

 apparatus for measuring the exhaustion by condensing a large quantity 

 of the residual gas into a small graduated tube, and then by allowing 

 calculation for the different pressure. 



The letters r, ^, p represent this apparatus : q is the small graduated 



* This tubing is specially made to stand great pressure by having a tube of canvas 

 between two tubes of india-rubber. 



2 f2 



