CHEMICAL BASIS OF THE ANIMAL BODY. 113 



which peptones similarly undergo during their passage through the 

 walls of the intestine into the neighbouring blood-vessels (see 309). 



3. Lactose (Milk-sugar). C 12 H 22 O n + H 2 O. 



It is found characteristically and solely in milk, in quantities 

 varying with the class of animal and at different times with the 

 same animal 1 . The percentage is relatively high in human milk. It 

 is also said to occur in the urine of lying-in women and sucklings 2 . 



Preparation. The casein is precipitated from diluted milk by the 

 addition of acetic acid. The nitrate from this is boiled to coagulate 

 the remaining proteids which are then removed by filtration. This 

 final filtrate is then concentrated and on prolonged standing yields 

 crusts of milk sugar which are purified by recrystallisation from hot 

 water. 



It yields, when pure, hard colourless crystals, belonging to the 

 rhombic system (four-sided prisms). It is less soluble in water than 

 dextrose, requiring for solution six times its weight of cold, but only 

 two parts of boiling, water ; it is entirely insoluble in alcohol and in 

 ether. It is fully precipitated from its solutions by the addition of 

 basic lead acetate and ammonia. 



Solutions of many metallic salts are readily reduced by boiling with 

 lactose, but the reducing power is less than that of dextrose. Thus 

 1 c.c. of Fehling's fluid which is reduced by 5 mgr. of dextrose requires 

 6 -7 mgr. of lactose provided that certain conditions as to the dilution 

 of the solution, duration of boiling &c. are attended to 3 . These are 

 important for the accurate volumetric estimation of lactose. The 

 specific rotatory power of lactose is (a) D = + 52*3, and is independent 

 of the concentration in solutions which contain up to 35 p.c. at 

 ordinary temperatures. Its rotatory power is thus identical with that 

 of dextrose. It is, however, readily distinguishable from dextrose by 

 its smaller solubility in water, insolubility in alcohol, and incapability 

 of undergoing direct alcoholic fermentation with yeast. It also does 

 not reduce Barfoed's reagent, and in this resembles maltose. When 

 boiled with dilute mineral acids it yields equal molecules of dextrose 

 and galactose (see p. 106), and since the specific rotatory power of the 

 latter of these is high [(a) D = + 83], this increase of rotatory (and 



1 See Gorup-Besanez, Lehrb. d. physioL Chem. 1878, S. 444. Konig, Chem. 

 d. mensch. Nahrungs- u. Genussmittel, 3 Aufl. 1889, Bd. i. S. 250 et seq. 



2 Hofmeister, Zt. f. physioL Chem. Bd. i. (1877), S. 101. See Neubauer u. 

 Vogel, Analyse d. Hams, 2 Theil, 1890, S. 48. 



8 Eodewald u. Tollens, Ber. d. d. chem. Gesell. 1878, S. 2076. Soxhlet, Zt. f. 

 prdkt. Chem. (2), Bd. xxi. 1880, S. 227. 



F. 



