280 PHYSIOLOGICAL CHEMISTRY 



into the weighing bottle 25 c.c. of distilled water at room tem- 

 perature, stopper the bottle, and weigh. 



Note : The pipette should be cleaned with cleaning fluid 

 and then thoroughly rinsed, first with tap water and then 

 once or twice with distilled water. If the liquid gathers 

 in drops on the inside of the pipette after its contents have 

 been allowed to flow out, the cleaning should be repeated. 

 On emptying a perfectly clean pipette or burette, the in- 

 strument should be so clean that no drops remain hanging 

 on the inside walls of the instrument; otherwise the 

 amount delivered will not correspond to the amount en- 

 graved on the glass. 



In filling a pipette, draw up the liquid into the stem, close the 

 top with the index finger, and allow the liquid to run out grad- 

 ually, until the bottom of the meniscus is level with the gradua- 

 tion on the stem. 



On emptying a pipette, allow the liquid to run out and drain 

 for about one-half minute. Then touch the tip of the pipette to 

 the surface of the liquid, or the side of the vessel. Do not blow 

 out the small amount of liquid remaining in the tip. This is the 

 method of emptying employed in the standardization of accurate 

 registered pipettes. In the case of accurate work, if only ordi- 

 nary pipettes or burettes are available, it is customary to stand- 

 ardize such instruments by weighing the amount of water they 

 will deliver and calculating the volume. 



Clean and dry the weighing bottle and measure into it from 

 the same pipette 25 c.c. of urine and weigh. Tabulate the 

 weights of (a) weighing bottle, (b) weighing bottle + water, 

 (c) weighing bottle + urine. Calculate the weights of water 

 and urine. The specific gravity of the urine then may be deter- 

 mined by dividing the weight of urine by the weight of water. 

 Since equal volumes of the two liquids have been weighed, it is 

 unnecessary that the exact volume delivered by the pipette be 

 known. Weighing the amount of water delivered by a pipette 

 or burette serves to determine the accuracy of such an instru- 

 ment, for a given weight of water corresponds, to a definite vol- 

 ume at a fixed temperature. In standardizing volumetric appa- 



