PIPETTES AND FLASKS. 



465 



Table CLIX. will give the figures obtained on a burette of fairly even 

 bore, but badly divided. 



TABLE CLIX. CALIBRATION OF BURETTE. 



III. Pipettes. Pipettes are used for measuring liquids by filling them 

 to the mark and letting the liquid run out ; the following points should 

 be noticed : 



(a) The bottom of the meniscus should coincide with the mark. 



(6) The pipette should be held vertically while it is running out. 



(c) The liquid should always be allowed to run out in the same manner. 



Perhaps the best manner of allowing the liquid to run out is to allow it 

 to flow as fast as possible, and, when empty, to touch the surface of the 

 liquid with the point and to withdraw it at once. It may, however, be 

 allowed to run out slowly, or a definite number of drops may be permitted 

 to run out after the main portion is delivered. Whatever method is adopted 

 during graduation must be strictly adhered to in practice. 



The graduation of pipettes is very simple : they are filled with water 

 as near 60 F. (15-5 C.) as possible, the contents run into a weighing bottle 

 and the water weighed. 



The pipettes should be each etched with a number and the weight of 

 water delivered tabulated for use. 



Pipettes used exclusively for delivering known weights of milk should 

 be graduated with milk of 1'032 specific gravity containing from 3-5 to 

 4-0 per cent. fat. In this case, the reading should be from the top of the 

 meniscus, as the lower edge is invisible. 



IV. Flasks. Flasks of capacity sufficiently small to permit of being 

 weighed when full, are filled with water as near 60 F. as possible, and 

 weighed. Each should be marked with a number, and the weight of water 

 contained by each tabulated. 



Larger flasks (e.g., litre flasks), if no balance sufficiently large be available, 

 are graduated by the following method : 10 successive portions of a little 

 less than 100 grammes of water at about 60 F. (15-5 C.) are weighed into 

 the flask (best from a 100 c.c. flask). A beaker containing a little water, 

 and a pipette are now weighed, and the litre flask filled to the mark by 

 water from the pipette ; the beaker, pipette, and remaining water are now 

 weighed : the difference between the weights plus the total weight of the 

 ten portions added together will give the weight of water in the litre flask. 



V. Leffmann-Beam or Gerber Bottles. These can be graduated with 

 sufficient accuracv by using each to make determinations of fat in several 



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