1874.] ijiJ [Koenig. 



part of the operator, hy \cMch the latter'' s attention is necessarily detracted 

 from the observation of the reaction. 



5. The apparatus must not come out of order easily under ordinary cir- 

 cumstances and attentive manipulation. 



6. The instrument must be applicable to all solutions used in volumetric 

 determinations. 



The present forms of the burette are of two types : a, the dropping^ bu- 

 rette, which in its simplest form is a graduated, lipped glass vessel, from 

 which the solution is poured out by the lip. 



Gay-Lussac improved this primitive instrument by the appendage of a 

 capilar tube, which although preventing a sudden stream, when but a 

 drop is wanted, still does not come up with the above given conditions 

 except the last, and is altogether an unhandy piece of apparatus. 



b, The valve burette. The very imperfect instrument just mentioned 

 stimulated invention, and we find as the next step the graduated tube 

 fixed to a stand vertically, and furnished with a glass ground perforated 

 stop-cock. This instrument is very nearly perfect, if well executed, but 

 from the nature of things it cannot fill the conditions 4 and 5. The rough 

 surface produced by grinding is exceedingly disposed to capilar action 

 and soon the effects from this show themselves by a layer of crystals ce- 

 menting the cock completely. These working defects are, however, so 

 well known, that I need hardly dwell any longer upon them. The same 

 applies to Geissler's glass-rod stopper. 



Then, Mohr showed how simply these difficulties could be overcome by 

 connecting the neck of the tube and the mouth with a piece of India rub- 

 ber tube pressed together by a spring clamp, or pinch cork. Comparing 

 this device with the 6 conditions, we find, after a long practice, that it is 

 far from being satisfactory. If the spring is strong it requires a remark- 

 able muscular exertion to open it, besides destroying the elasticity of the 

 rubber ; if weak, it will not close the valve completely. I find, moreover, 

 that the rubber tube becomes soon deteriorated chemically, especially by 

 alkaline solutions, and that many volumetric solutions cannot be brought 

 into contact with such a large surface of rubber without undergoing a 

 change in their docimastical value. The substitution of Hoffmann's 

 screw clamp for the spring clamp is not so very happy ; it requires both 

 hands for the adjustment just in the moment when one hand is most 

 needed for stirring the liquid, besides it acts too slowly, several turns of 

 the screw being needed to overcome the elasticity of the comparatively 

 thick rubber tubing. 



J. Blodget Britton described an apparatus (Journal of the Franklin In- 

 stitute, 1870,) which is undoubtedly a considerable step forward. He re- 

 cognized that the valve had to be placed externally, and that it had to 

 possess a screw movement. He draws his burette at the lower end into 

 a capilar tube, bends it slightly, so as to bring the orifice in contact with 

 a cork plate, which itself is fastened to a steel spring, opened by a screw 

 bolt. To prevent splashing, the opening must be very narrow, and con- 



