570 APPENDIX. 



beaker, and the last few drops removed from the point of the pipette by 

 touching it against the side of the beaker. 



/ii/n'tfi'n. These are used for delivering standard solutions. There 

 nil forms of burette, but the most convenient is that of Mohr. 

 It consi-ts of a graduated tube, to whose lower end an India-rubber tube 

 is attached, which can be opened and shut by a spring clip (fig. 849), so 

 that the operator can let the solution run out or stop it at will. The bu- 

 rette is supported in an upright position on a stand made for the purpose 

 .V2). To prevent dust getting in, a polished marble should be 

 placed on its upper end. Iii man}' cases the spring clip answers well, 

 but when nitrate of mercury is used it attacks the clip, and bichromate 

 of pota-h destroys the India-rubber. For such liquids a burette fur- 

 ni-lied with a glass stopcock is to be preferred. A burette should be 

 filled by allowing the Tumid to How gently into it while it is held in an 

 inclined portion in the hand till it stands above the zero mark. The in- 

 stnnnent is then replaced. If any air-bubbles arc present, they must be 

 allowed to break, or removed by a glass rod. The solution is then al- 

 lowed to flow out till its level corresponds to the zero mark on the burette. 

 " fl Burettes and other Graduated Instruments 'used in 

 >h/xi*. When liquid is contained in a narrow tube, its 

 surface is higher at the edges where it touches the glass than elsewhere; 

 and if we examine the curved surface by transmitted light, it seems to 

 be formed nt' several /.ones or bands, the lowest of which is dark (fig. 

 :>.">()). To avoid errors and uncertainty, the under border of the dark 

 arded as indicating the level at which the liquid stands. 

 In reading, the eye must of course be exactly level with the surface, 

 otherwise the reading will be either too high or too low. The under 

 surface of the liquid is more easily seen if a card, with its under half 

 blackened, while its upper half remains white, be held behind the liquid, 

 so that the division between the black and whiteparts is about one-eighth 

 of an inch below its surface. The lower surface of the liquid then seems 

 to be bounded by a sharp black line (Sutton). Burettes may be read 

 very ea-ily and with great accuracy by tiding Krdmann's float (fig. :!.">r. 

 This is an elongated gla<s bulb, weighted with mercury at its lower end. 

 so that It fioate Upright. Its diameter being a very li'ttle less than the 

 calibre of the burette which contain* it, it moves freely, but at the same 

 time steadily, up and down. A horizontal mark round its middle is 

 taken as indicating the height at which the liquid stands, the absolute 

 height beiiiLT disregarded. 



Lit> i lie solution used in the neutralization of albumi- 



iquids is prepared by dissolving a little litmus in distilled water, 

 decanting the liquid from the sediment, and diluting it as required. For 

 determinations, of the str-ngUi of acid, the litmus solution is made b\- 

 putting 10 grammes of solid litmus into half a litre of distilled water, let- 

 ting it >tand for a few hours in a warm place, decanting the clear fluid, 

 adding a few drops of dilute nitric acid so as to produce a violet color, 

 and preserving it in an open bottle with a narrow neck. If the color 

 should at any time partially disappear, it may be restored by exposing the 

 liquid to the air in an open bottle (Sutton)/ 



fini, ,,f .s',,^ ( /._Kjll ;i burette with solution of soda, 

 and cautiously drop this into (\.'.\ grammes of purified oxalic acid in crys- 

 :ite dry lint not elllore-ced. dissolved in about ?() c. c. of distilled 

 water, until the acid is exactly neutralized, as indicated by litmus. Note 

 the number of irrain measures (n) of soda solution used, and having 

 then introduced '.100 c. c. of it into a graduated jar. augment this quantity 



by the addition of water until it becomes c . c> ]f. for example 



>i=!i:j. the 1)00 cub. cent, should be augmented to 90 x > 10() = 967.7 



93 



