826 



SILVER ASSAYING 



pipe T enters with friction. It is by this pipe that the air is sucked into the pipette, 

 when it is to be filled from its beak. 



The pipette is supported by two horizontal arms u K (fig. 1845) moveable about a 

 common axis, A A, and capable of being drawn out or shortened by the aid of two 

 longitudinal slits. They are fixed steadily by two screw-nuts, e e', and their distance 

 may be varied by means of round bits of wood or cork interposed, or even by opposite 

 screw-nuts, o o'. The upper arm H is pierced with a hole, in which is fixed, by the 

 pressure of a wooden screw v, the socket of the pipette. The corresponding hole of 

 the lower arm is larger ; and the beak of the pipette is supported in it by a cork stop- 

 per, L. The apparatus is fixed by its tail-piece p, by means of a screw to the corner 

 of a wall, or any other prop. 



The manner of filling the pipotte is very simple. We begin by applying the fore- 

 finger of the left hand to the lower aperture, c ; we then open the two stopcocks E and 

 E'. Whenever the liquor approaches the neck of the pipette, we must temper its 

 influx, and when it has arrived at some millimeters above the mark a b, we close the 

 two stopcocks, and remove our forefinger. We have now nothing more to do than 

 to regulate the pipette; for which purpose the liquid must touch the line a b, and must 

 simply adhere externally to the beak of the pipette. 



This last circumstance is easily adjusted. After taking away the finger which closed 



1847 



1845 



1846 



the aperture c of the pipetto, we apply to this 

 orifice a moist sponge, m, fig. 1846, to absorb 

 the superfluous liquor as it drops out. This 

 sponge is called the 'handkerchief' by M. Gay- 

 Lussac (mouchmr). The pipette is said to be 

 wiped, when there is no liquor adhering to its 

 point exteriorly, 



For the convenience of operating, the handker- 

 chief is fixed by friction in a tube of tin-plate, 

 terminated by a cup, open at bottom to let 

 the droppings flow off into the cistern c, to 

 which the tube is soldered. It may be easily removed for the purpose of washing it ; 

 and, if necessary, a little wedge of wood, o, can raise it towards tho pipette. 



To complete the adjustment of the pipette, the liquid must be made merely to descend 

 to the mark a, b. With this view, and whilst the handkerchief is applied to the beak 

 of the pipette, the air must be allowed to enter very slowly, by unscrewing the plug v, 

 fig. 1843 ; and at the moment of the contact, the handkerchief must be removed, and the 

 bottle F, destined to receive the solution, must be placed below the orifice of the pipette, 

 fig. 1846. As the motion must be made rapidly, and without hesitation, the bottle is 

 placed in a cylinder of tin-plate, of a diameter somewhat greater, and forming one 

 body with the cistern and the handkerchief. The whole of this apparatus has for a basis 

 a plate of tinned-iron, moveable between the wooden rollers R H, one of which bears a 

 groove, under which the edge of the plate slips. Its traverses are fixed by two abut- 

 ments, b b, placed so that when it is stopped by one of them, the beak of the pipette 

 corresponds to the centre of the neck of the bottle, or is a tangent to the handkerchief. 

 This arrangement, very convenient for wiping tho pipette, and emptying it, gives tho 

 apparatus sufficient solidity, and allows of its being tnkcn away, and replaced without 

 deranging anything. It is obvious that it is of advantage, when once the entry of the 

 air into the pipetto has been regulated by tho screw v. to leave it constantly open, be- 

 cause tho motion from the handkerchief to the bottle is performed with sufficient 

 rapidity to prevent a drop of tho solution from collecting and falling down. 



