ALDEHYDE FIGURE. 183 



which would be equivalent approximately to 0-19 for total 

 proteins. 



The author much prefers strontia, and has proved that the 

 method has considerable accuracy. 



The acidity and aldehyde figure usually approximate in fresh 

 milk to the same figure, it being only rarely that the difference 

 amounts to more than 2 ; in certain abnormal samples, usually 

 low in solids not fat, the acidity is much lower than the aldehyde 

 figure, and in these the factor 0-170 for calculating proteins is 

 not exact. 



Estimation of Lecithins. Bordas and de Raczkowski deter- 

 mine lecithins in milk thus : 100 c.c. of milk are shaken with 

 100 c.c. 95 per cent, alcohol, 100 c.c. of water and 10 drops of 

 acetic acid. The precipitate is extracted with three successive 

 quantities of 50 c.c. each of hot absolute alcohol. The mixed 

 extracts are evaporated, and the residue taken up with a small 

 quantity of a mixture of equal parts alcohol and ether ; the ether 

 is evaporated, and the residue saponified by potassium hydroxide, 

 and the soap solution acidified with dilute nitric acid. The fatty 

 acids are filtered off, and the filtrate evaporated to dryness, 

 mixed with 10 c.c. of strong nitric acid, and oxidised completely 

 by the addition of potassium permanganate little by little. A 

 few drops of sodium nitrite solution (1 : 10) are added to dissolve 

 the manganese hydroxide, and the nitrous acid expelled by boiling. 

 The phosphoric acid is precipitated with ammonium molybdate 

 and estimated as magnesium pyrophosphate. The weight of 

 Mg 2 P 2 7 multiplied by 1-5495 will give the glycero-phosphoric 

 acid, and by 7-27 the lecithins. 



Buron's method as modified by Brodrick-Pittard consists in 

 dropping the sample into a mixture of equal parts of alcohol 

 and ether acidified with acetic acid, evaporating the filtrate at 

 a low temperature, adding anhydrous sodium sulphate, grinding 

 and extracting with dry ether and determining the phosphorus 

 in the ethereal extract. 



Estimation of Catalase. When hydrogen peroxide is added 

 to milk, the catalase in the milk splits it up into water and oxygen, 

 which is given off as gas ; as, however, this is partly in a super- 

 saturated solution, as a good deal of frothing occurs, and as the 

 reaction is not a finite one, it is difficult to measure the total 

 amount of oxygen given off accurately, and the determination 

 is complicated by the fact that the rate of evolution varies 

 according to the temperature. 



The most accurate method of measuring the catalase is that 

 due to Faitelowitz, who places 100 c.c. of milk, as well as a tube 

 containing 1 to 2 c.c. of hydrogen peroxide of known strength, 

 in a flask holding 115 to 125 c.c., which is connected to a gas 



