Brltton.] 



194 



[Dec. 3, 



pv wTjlJ^-. 



and afterwards to be as conveniently replaced. Tlio old arrangement (Fig. 



4) required two screws, for the removal of 

 which a screw driver was necessary. I, how- 

 ever, have not adopted the new to the com- 

 plete expulsion of tiie old, Avhich costs but a 

 trifle, and in some respects is excellent. 



Notes. Each of these burettes, it will be 

 observed, has its graduated scale on white pa- 

 per firmly pasted against the upright and be- 

 hind the tube, and not cut on the tube itself; 

 the graduation conseciuently is as distinct as it 

 is possible to make it, and the reading being 

 over the fluid, which affords a level to guide 

 the eye, is done with the least liability to er- 

 ror. In this manner the finest of my burettes 

 have been made. When the cutting is on the 

 tube, I prefer it to be behind, and not in front, 

 and well rubbed with lampblack and oil, and 

 have a white paper pasted on the upright 



for a ground to make it more plainly seen. 



Each tube has its lower end drawn out small enough to just allow a stout 

 hog- bristle to pass freely in and out of the vent, and in order that the dis- 

 charge may be made sufticienlly far over into the receiving vessel, it is 

 bent outwards with the point downwards, so that the vent shall be an inch 

 and a half from the upright, and three inches from the bottom of the stand. 

 The rod-stoppered tubes have their points made smooth bj^ fusion, but the 

 spring valved have theirs carefully ground on an emery plate to allow the 

 valves to shut quite true and tight with the line of contact just vertical, and 

 not at an angle. 



Unnecessary thickness of glass about the points is avoided, and the valves 

 are made small, especiallj' at their lower ends, to give as little surface of 

 material as possible for the retention by capillary attraction of the drop- 

 ping fluid. To further obviate such retention, my practice is to occasionally 

 rub the points and valves with a little pure tallow, the effect of which is 

 excellent. Burettes made in the manner described, may with their valves 

 wide open be discharged of 100 cc in about two minutes. But when there 

 is a rapid discharge, a considerable quantity of the fluid is held back on the 

 walls of the tube by attraction, and the quantity so held becomes increased 

 with increased rapidity of discharge; this is shown most distinctly when a 

 strong solution of chameleon is used, for the material gives color to the tube 

 80 long as any of it remains. A rapid discharge consequently will not per- 

 mit a prompt reading, and if the tube be long some minutes must elapse 

 before all of a fluid, of any considerable density, reaches the bottom. 

 The case, however, is different when the dis(;harge is moderately slow and 

 uniform; the reading may then be done usually soon after the reaction has 



