PHOSPHORIC ACID IN BLOOD 181 



tubes or fittings sufficiently alike so that when both tubes are 

 filled with the same solution and the standard set at a given point 

 say 25 mm., the test solution will read 25 mm. Kober's suggestion 

 has always been followed in this connection; i.e., set the test 

 solution at the required point (say 25 mm.) and adjust the standard 

 until the two fields are the same. Then set the standard at that 

 point and call it 25 mm. The correctness of the position should 

 be tested frequently. 



The Richard's type of nephelometer (with moving jackets), 

 made by adapting a Duboscq colorimeter, is believed to be pref- 

 erable to the plunger type, because no foreign body with its pos- 

 sible sources of contamination comes in contact with the solutions. 

 The danger will be realized when attention is directed to the fact 

 that in ordinary determinations the amounts measured are about 

 0.03 mg. of HsPO4 in 50 c.c. of solution. 



Corrections. The readings obtained in the nephelometer are 

 not exactly proportional to the amounts of phosphoric acid in 

 the solutions. Strong solutions appear stronger and weak solutions 

 weaker than they should. Different schemes for correction of 

 the readings have been suggested, none of which has been found 

 entirely satisfactory and for exact work the necessity for cor- 

 rection is avoided by adjusting the strength of the standard until 

 the correction falls within the limit of error of the method. Solu- 

 tions up to about 25 per cent stronger or weaker than the standard 

 may be compared without correction. Solutions from 25 to 50 

 per cent above or below the standard may be read with correction 

 with an accuracy of 3 to 5 per cent of the theoretical value. It 

 is not thought advisable to attempt comparison of solutions vary- 

 ing more than 50 per cent from the standard. The standard phos- 

 phate solution contains 0.03 mg. of HsPCU per c.c. and if the regular 

 5 c.c. make a standard more than 50 per cent above or below the 

 test solutions, a fresh standard should be made with 2 to 10 c.c. 

 of the standard, which will give solutions falling within the ordinary 

 range of blood phosphates. Adjustment of the salt content must 

 of course be made as with the ordinary standard. 



In the determination of lipoid-phosphoric acid it makes no 

 difference whether the second part of the extraction the heat- 

 ing is done at once or after some time. Ordinarily it has been 

 found convenient to keep the flasks in a cool, dark place for a week 

 or more until a suitable number have accumulated before finish- 



