14 XNTRODDCTORY — THE CONSTITUENTS OF MILK. 



•often have the face at the end of the wedge greatly prolonged. 

 The ^-modification crystallises in needles. 



The taste of the a-modification is not sweet, and from its 

 comparative insolubility it appears to be gritty. In solution 

 milk-sugar has a sweet taste of about a quarter the sweetness 

 of cane-sugar. 



As already stated, on heating to 170° C. it turns brown, and 

 lacto-caramel is formed ; a similar change takes place by heating 

 an aqueous solution to 100° C. for some hours ; the presence of 

 small amounts of alkali greatly increase the browning of the 

 solution. The rotatory power is greatly diminished. 



Chemical Properties. — Milk-sugar, in common with other 

 aldoses and ketoses, reduces alkaline solutions of copper, silver, 

 and mercury, forming cuprous oxide, and metallic silver and 

 mercury respectively. On this fact the well-known Fehling's 

 test for sugar is based. The amount of reduction is constant for 

 fixed amounts of milk-sugar under the same conditions, and is 

 nearly proportional to the amount of milk-sugar. Each sugar 

 shows a definite amount of reduction in the same way, -and a 

 valuable method for distinguishing them is thus available. The 

 difference between reduction by various sugars is not due to any 

 difference in the reaction with the metallic salt, but depends on 

 their relative stability towards alkalies. 



On warming with dUute nitric acid (sp. gr. 1'2) an energetic 

 action takes place, mucic acid, together with saccharic, oxalic, 

 and other acids being formed ; the mucic acid, which can be 

 separated by its relative insolubility, amounts to about 32 per 

 cent, of the weight of the milk-sugar. This is due to the galactose 

 portion of the milk-sugar. Strong nitric acid (sp. gr. 1 '5) mixed 

 with sulphuric acid, to absorb the water formed in the reaction, 

 gives rise to the formation of tri- and penta-nitrates ; both 

 these compounds have explosive properties. The penta-nitrate 

 is a constituent of certain high explosives. 



On heating with an excess of precipitated copper oxide gummy 

 acids are formed, such as galactinic and pecto-galactinic acids, 

 compounds which are also formed from galactose. 



By oxidation with bromine lacto-bionic acid is formed, in 

 which the COH group is converted into COOH. Potassium 

 permanganate in acid solution oxidises it to carbonic acid, but 

 the reaction is not complete, not more than 80 per cent, of the 

 theoretical quantity of carbon dioxide being obtained. 



By heating with phenylhydrazine acetate two compounds are 

 formed ; one of these— phenyl-lactosazone— is sparingly soluble 

 in cold water, but in 80 to 90 parts of hot water, from which it 

 separates on cooling in fine yellow needles melting at 200° C. with 

 decomposition. It is also soluble in alcohol and ether ; the latter 



