45 



VII. 



QUANTITATIVE ANALYSIS. 



Centrifugal Method. The centrifuge as ordinarily used gives 

 the hulk percentage of the constituents, this percentage being 

 arrived at arbitrarily as a result of numerous determinations 

 upon normal urine. Knowing the normal bulk percentage it is 

 not difficult to determine an abnormal or 

 pathologic amount of a substance as indi- 

 cated by its excess or deficiency, and the 

 method is therefore a convenient and 

 expeditious one for clinical purposes. 

 To a beginner, however, the method as 

 commonly employed, is misleading in 

 that the bulk percentage and the true or 

 actual percentages are widely different. 

 Take for example the phosphates of the 

 urine. The normal hulk percentage as 

 given by the centrifuge is 8% for man; 

 1% or less for the horse. Whereas, the 

 real amount of phosphates present as 

 P^Og in the urine of man for the whole 

 24 hours (1500 cc. of urine) is only 

 about 3.5 grams, or in 1000 cc. about 2.5 

 grams, or a true percentage of 0.25%. 

 The true percentage may be calculated 

 from the centrifuge by giving to each 0.1 cc. of precipitate a de- 

 termined value with reference to the amount of PoO.^ present. 



Each centrifuge tube is graduated to 15 cc. The first 10 cc. 

 are divided so that each cubic centimeter is divided into 10 parts, 

 each part representing 0.1 cc. The remaining 5 cc. are for hold- 

 ing the test reagents which are added to the 10 cc. of urine. The 

 first cubic centimeter because of the tapering end, will admit of 

 finer graduation than the remainder of the tube. The first half 

 of the cubic centimeter is divided into ^/^o cc. (or .025 cc.) in 

 order that small amounts of precipitate may be accurately read ; 

 the second half of the cubic centimeter is divided into V20 cc. (or 

 .05 cc.) ; while the remaining 9 cc. are each divided into Vio cc. 

 (or 0.1 cc.) as above stated. It is convenient to take the ^/lo cc. 

 as the unit in the calculation of the true percentage. By taking 



Fig. 9. 

 Hand Centrifuge. 



