12 INTKODUCTORY — THE UONSTITUENTS OF MILK. 



It is seen that tlie rotation is approximately constant for the 

 first 6 minutes, and averages 12-75°, which corresponds to 

 [ajn = 83-99°. After 24 hours the rotation is constant at 7-95°, 

 which corresponds to [a]^ = 52'37°. 



The figures given in the " calculated " column are deduced by 

 the formula — 



logio (Rt - R ») = 0-68124 - 0-00491 (T - 6). 



The fact that the fall in rotation is expressed by a logarithmic 

 curve shows that the rate of change is proportional to the amount 

 of imchanged substance in solution ; this is Harcourt's law of 

 mono-molecular change. 



The ratio between the initial rotation and the final rotation, 



83-99 

 which may be called the bi-rotation ratio, is -„ „„ = 1-604. 



The mean of several determinations has led to the value for 

 [aJD of the hydrated a-modification = 84-0°, and the bi-rotation 

 ratio 1 '6 ; these are the figures given by Schmoeger, who, how- 

 ever, assigned to them an approximate value only. 



The small amount of thermal change during change of rotation 

 and absence of change in density and freezing point show, with 

 a considerable degree of probability, that the change manifested 

 by alteration in rotation is intra-molecular. It is probably 

 caused by the migration of the water of hj'dration from one 

 carbon atom to another. 



The anhydrous modification of the a-modification is obtained 

 by heating the hydrated modification to 130° C. It is hygro- 

 scopic and dissolves in water with evolution of heat ; the solu- 

 bility is much greater than that of the hydrated modification. 

 The optical properties are stated by Schmoeger to be the same 

 as those of the hydrated modification. 



Both the a- and /3-modifications are converted on dissolving 

 in water into a stable equilibrium form. Schmoeger gives the 

 specific rotatory power [ajj, as 52-53° at 20° C, diminishing 

 0-075° for each degree C. above and this increasing for lower 

 temperatures. The author can absolutely confirm these numbers. 

 It has never been prepared pure in the solid state, though con- 

 siderable evidence of its existence, both in the hydrated and 

 anhydrous modifications, has been obtained by the author. 



By the addition of alcohol or, better, ether to a very highly 

 supersaturated hot solution of milk-sugar, it sets to a solid mass, 

 which may be dried in vacuo, and does not then lose Aveight at 

 100° C, but which contains, however, a certain amount — 2 to 4 

 per cent. — of water of hydration which is lost at 130° C. There 

 is no appreciable change in rotation on dissolving this product 



