OF ARTS AND SCIENCES. 5 



the gas moved forward past N to the aspirator 0,* and then escaped. 

 The bottle L, of about 10 litres' capacity, sometimes two bottles eacli of 

 that volume, connected between G and N', served to reduce the rate of 

 pressure fluctuations arising from irregularities in the aspiration. At 

 JV was connected the pressure-regulator, which I will describe in 

 detail, as it is so far as I know wholly new and is available for many 

 purposes. 



The aspirator under a given head of water can exhaust at a definite 

 rate, maintaining a certain exhaustion ; if the head be increased or 

 lessened, the rate of exhaustion will be increased or diminished, and 

 thus if the leakage into the exhausted space, as through the capillaries 

 in this case, be sensibly constant, the pressure in the exhausted space 

 must diminish or increase. It is often impracticable, as it was for 

 me, to maintain a constant head of water, and some means is there- 

 fore necessary to overcome this irregular action. The arrangement 

 used was that shown in the sketch. A large glass tube M, open at 

 both ends, stands upright in an open mercury trough of considerably 

 larger dimensions. Into its top is inserted a rubber stopper with two 

 borings, through one of which passes one arni of a T joint whose 

 other two arms are connected respectively with the aspirator and the 

 vessel to be exhausted. Through the other boring passes a tube of 

 1 mm. to 2 mm. diameter bent twice at right angles, and dipping 

 into the mercury in the trough. Suppose that the vessel to be ex- 

 hausted is closed — the apparatus covers cases ranging from this to 

 a leakage of about half the rate of exhaustion of which the pump is 

 capable — and the aspirator set in operation. As the exhaustion 

 proceeds, the mercury rises equally in both large and small tubes 

 until the mercury in the trough drops below the point of the latter, 

 whereupon, as soon as the excess of external pressure is sulficiently 

 great to overcome the friction of the mercury, the column in the fine 

 tube rises rapidly and flows over into the upper part of the large tube 

 and upon the mercury surface in it. This overflow is of course im- 

 mediately followed by an ini-ush of air through tbe open point, and a 

 consequent sudden lowering of the mercury in the large tube with a 

 corresponding rise of level in the trough which closes or partly closes 

 D. This first sudden action is followed by several of lessening vio- 

 lence, and a steady condition is soon reached, in which there is a 

 continuous inflow of air and small drops of mercury at the point, the 



* Richard's jet aspirator was used. See Amer. Jour. Sci., [3], viii. 200 ; 

 Chem. News, xxxiv. 141; Trans. Amer. Inst. Mining Eng., vi. 492 (1879). 



