DUCLAUX'S METHOD. 237 



air passed through for some hours. After cooling, the tube is 

 weighed and the loss noted as water. 



The fat is now extracted by carbon bisulphide (other solvents, 

 such as ether or chloroform, may be used), the tube again dried 

 and weighed, and the amount of fat deduced by difference. 



The tube may be similarly exhausted by alcohol, hot or cold 

 water, and the loss of weight noted after each extraction. 



Ash and Salt. A fresh portion of cheese is weighed out into 

 a platinum basin, and ignited to obtain the ash ; in this, the 

 chlorine is titrated with standard silver nitrate, using potassium 

 chromate as indicator. 



Proteins and Products of Ripening. About 10 grammes of 

 cheese are weighed and mixed intimately in a mortar with 

 about 10 c.c. of water ; a very homogeneous paste is formed, and 

 this is left for half an hour to ensure the perfect contact of the 

 water with the solid matter. More water is added, little by 

 little, the mixing in the mortar being continued till 100 c.c. have 

 been added. The mixture is now filtered through a porous 

 porcelain filter by means of reduced pressure ; in several hours 

 60 to 70 c.c. can be obtained. 



Ten c.c. are evaporated in a platinum basin, the residue dried 

 at 100, weighed, then ignited, and the ash weighed. The 

 difference will give the organic matter ; this is termed by Duc- 

 laux " caseone," and represents the products of ripening. The 

 percentage may be calculated with approximate accuracy, by 

 multiplying by 100 + the weight of water in the amount of 

 cheese taken, and dividing by one-tenth of the weight of the 

 cheese. 



The remainder of the filtered liquid (50 c.c.) is brought, by 

 the addition of water, to 150 c.c., and distilled into standard acid, 

 to determine the free ammonia ; this determination is not very 

 exact as ammonia is gradually liberated as the distillation pro- 

 ceeds ; hence it is usual to stop the distillation when 75 c.c. 

 have distilled over. A little calcined magnesia suspended in 

 25 c.c. of water is next added, and about 50 c.c. are distilled into 

 standard acid for the estimation of combined ammonia. 



The residue in the distilling flask is rendered acid by the addi- 

 tion of a little sulphuric acid, and made up to 55 c.c. ; 40 c.c. 

 are distilled off, and the volatile acid received in standard alkali. 

 The acid is calculated as butyric by multiplying the number 



N 

 of c.c. of alkali used by the factor 0-00975 (this factor assumes 



that 90-2 per cent, of the total acid will be obtained under these 

 conditions). 



