PHYSIOLOGICAL CHEMISTEY 337 



(a) chlorides by silver nitrate white precipitate insoluble in 



nitric acid ; 



(b) phosphates white precipitate on addition of ammoniacal 



magnesium citrate solution. Filter off this precipitate. Dis- 

 solve in nitric acid, and heat with nitro-molybdic acid 

 yellow precipitate ; 



(c) sulphates white precipitate with barium chloride, insoluble in 



hydrochloric acid. 



In all three tests phosphates are precipitated, but in (a) they are 

 soluble in nitric acid, in (c) they are dissolved by hydrochloric acid. 

 (Cf. salts of urine.) 



The amount of sulphate present is usually very small. This filtrate 

 may also be tested for sugar by Fehling's or Nylander's test. 



The Estimation of Sugar in Blood. To estimate sugar in blood it is 

 necessary that the proteins and haemoglobin be removed. This is most easily 

 done by Waymouth Reid's method. Into a beaker of about 600 c.c. capacity are 

 placed 250 c.c. of a 7% solution of phospho-tungstic acid containing 2 % HC1 

 and the whole is weighed. The blood is then added, the contents well stirred, 

 and the beaker again weighed. The difference in weight gives the amount of 

 blood added. The beaker is then heated on a sand bath (or better still an oil 

 bath), its contents being meanwhile briskly stirred. The proteins including 

 the haemoglobin are, by this treatment, precipitated and form at first a brown 

 gummy mass floating in a clear liquid. After a little the coagulum becomes 

 brittle and sinks to the bottom of the beaker. Great care must now be taken 

 that the beaker does not crack. When all the coagulum has settled to the 

 bottom, the beaker is cooled and the supernatant fluid filtered through paper 

 into an evaporating dish, the paper well washed into the same dish, the contents 

 of the latter nearly neutralised with NaOH, but left faintly acid, and the 

 evaporating dish then placed on a boiling water bath. 



While the above fluid is evaporating the brittle protein precipitate is removed 

 from the beaker to a mortar, ground up with some water til a chocolate-like 

 paste is obtained and then washed on to a large suction filter plate and sucked 

 dry. It is washed with water three times. The washings are then transferred 

 to a 2 litre flask, nearly neutralised and boiled down to a small volume (50 c.c.) 

 with the flask on the slant. The evaporated washings are then mixed with the 

 contents of the evaporating dish (the evaporated supernatant fluid) and the whole 

 brought to a volume of 50 c.c., after which it is cooled, almost neutralised, 

 filtered through a small filter paper, the filter washed and the volume of the 

 filtrate and washings brought up to 100 c.c. The sugar is then estimated in this 

 by one of the methods described on page 450. 



Blood Plasma. All the above bodies are present in plasma, which 

 contains one substance in addition, namely, Fibrinogen. We have 

 already shown this (Expt. III. d). Plasma, however, does not contain 

 thrombin ; serum does. 



When we remember the function of the blood it is obvious that 

 there are many bodies other than the above present in both plasma 



