CHK.MICAL U.VSIS OF THE ANIMAL BODY. 113 



through the walls of the intestine into the neighbouring blood- 

 vet- fj :)U!). 



3. Lactose (Milk-sugar). CuH^On + H,O. 



It is fouiul characteristically and solely in milk, in (juautities 

 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. - 



I' i-' / 1" ration. The casein is precipitated from diluted milk by 

 the addition of acetic acid. The filtrate fmm this is boiled to 

 coagulate, the remaining proteids, which are then removed by fil- 

 tration. This final filtrate is then concentrated, and on prolonged 

 standing yields crusts of milk sugar which are purified by recrys- 

 tallisation fmm hot water. 



It yields, when ]>ure, hard colourless crystals, belonging to the 

 rh>mbic 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 Mid 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 thud which is reduced by 5 mgr. of dex- 

 trose requires 6'7 m.^r of lactose provided that certain conditions 

 as to the dilution of the solution, duration of boiling, \-e.. are 

 attended to. 3 These are important for the accurate volumetric 

 estimation of lactose. The specific rotatory power of lactose is 

 (a) D =-r-52'3, and is independent of the concentration in solu- 

 tions which contain up to 35 p. c. at ordinary temperatures. Its 

 rotatory power is thus identical with that of dextrose. It is, how- 

 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 

 I'.aifoed's reagent, and in this resembles maltose. When boiled 

 with dilute mineral acids it yields eipial molecules of dextrose and 

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

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

 (and reducing) power on treatment with acids affords a further 

 convenient means of discrimination between lactose and dextm>e. 



1 Sec <;.>rnp Hcs.-iiK'/., f.flir/,.,1 /./,- ,W .<"/,, I'hnn d. mensch. 



Xillinni./*- u. <;,nnsxmittrl,:\ Allll (I-VI). I'ul. I. S. L'.'.O./ s/. 



: Ilufmfintcr, /J /". f,lti/Ml. ('In m Mil. i. (1877), S. loi. Sco Ncubauor u. Vogel, 

 Analyse </. //nrns, li'Td.-il. I- n>. s I- 



..l.-w.-iM u. T..I1.-!,.. /'< . '/ / chem. Getell. 1878, S. 2076. S.xhk-t, /A. f. 

 prakt. Chtm. (-2), IM \\i I -MI. s 



8 



