718 METABOLISM AND ANIMAL HEAT 



mark. Allow the blood to run back just to the mark, and immediately 

 discharge the contents of the pipette into a large test tube (about 50 c.c. 

 capacity) containing 8 c.c. of distilled water. Shake until haemolysis 

 is complete. Then add 15 c.c. of saturated aqueous solution of pure 

 picric acid ; shake thoroughly to precipitate the proteins of the blood, 

 and filter through a small filter paper. Into each of two long narrow 

 test tubes (capacity about 25 c.c.), which have been marked accurately 

 with a file at 10 c.c., measure 7 c.c. of the filtrate, add 2 c.c. of the 

 saturated solution of picric acid and i c.c. of a 10 per cent, solution of 

 anhydrous sodium carbonate. A duplicate determination w r ill thus be 

 made. The test tubes are placed in an autoclave (according to Pearce's 

 modification of the method), and the pressure gradually brought up to 

 20 to 25 pounds to the square inch (2-5 kilogrammes to the square centi- 

 metre), and kept at this level for twenty-five minutes. Picramic acid is 

 formed by the reducing action of the sugar, and from the amount of 

 picramic acid estimated by a colorimeter the amount of sugar is de- 

 duced. Let the autoclave cool down till the pressure is zero, remove 

 the tubes and allow them to cool to room temperature. Sufficient 

 water is now added to each tube to bring the volume back to the 10 c.c. 

 mark, and after shaking the contents are filtered through cotton into 

 the (Duboscq) colorimeter bottles. The determination is made by 

 comparison with a solution containing a known amount of pure dex- 

 trose which has been carried through the same process, or with a 

 picramic acid standard solution.* 



A solution of pure dextrose may be used as a standard instead of the 

 picramic acid, as follows : Into a test tube place 4 c.c. of a solution, 

 corresponding to 0-56 milligramme dextrose; add 5 c.c. of the saturated 

 picric acid solution, and i c.c. of 10 per cent, solution of anhydrous 

 sodium carbonate. Place in the autoclave and proceed as above 

 described, bringing the final volume again to 10 c.c., and filtering 

 through cotton into one of the colorimeter bottles. 



Calculation : 



Reading of standard mgm. dextrose in standard^ mgm. dextrose per 

 Reading of unknown c.c. blood used J~ c.c. blood. 



The amount of blood taken for each of the duplicate determinations 

 is x 2 c.c. =0-56 c.c. The standard=o - 56 milligramme dextrose in 

 10 "c.c., so that the second fraction on the left side of the equation 

 becomes unity, and the reading of the standard divided by the reading 

 of the unknown gives at once the number of milligrammes of dextrose 

 in i c.c. of blood. 



5. Milk. (i) Examine a drop of fresh cow's milk with the micro- 

 scope. Note the fat globules of various sizes. 



(2) Determine the specific gravity of the milk with a hydrometer 

 (lactometer). Then centrifugalize some of the milk to separate the 

 cream, which rises to the top of the tubes. Remove the cream and 

 determine the specific gravity of the skimmed milk. It will be found 

 to have increased, since the fat is of lower specific gravity than the 

 rest of the milk. Normal cow's milk has a specific gravity of 1,028 

 to 1,034, skimmed milk 1.033 to 1,037. 



* Picramic acid, 56 milligrammes, anhydrous sodium carbonate, 100 

 milligrammes, and distilled water to make up 1,000 c.c. Dissolve the sodium 

 carbonate in about 50 c.c. of water, add the picramic acid and dissolve it 

 with the aid of heat. When cooled to room temperature add enough water 

 to make 1,000 c.c. Picramic acid obtained on the market varies in the amount 

 of colour produced in the preparation of this solution. The standard should 

 therefore be compared with a solution of pure dextrose which has been standard- 

 ized by another method, and then treated by the Lewis and Benedict method. 



