156 H. S. H. WARDLAW. 
From these figures it will be seen that there is no definite 
diminution in the depression of the freezing point after the 
removal of various amounts of suspended matter from milk, 
indeed there seems to be a tendency for the depression of 
the freezing point to increase eventually. Any effect due 
to the removal of adsorbed electrolytes in the deposit is 
therefore within the experimental error in the present 
case. 
It was thought that the tendency for the depression of 
the freezing point to increase might be due to an increase 
in the number of particles present in solution brought about 
by the souring of the milk, by the breaking down of lactose 
into lactic acid. The splitting of lactose takes place in 
two main stages, (1) it is hydrolysed by lactase into glucose 
and galactose, (2) the hexoses thus formed are converted 
into lactic acid. Hach molecule of hexose gives rise to two 
molecules of lactic acid, so that in all four molecules of 
lactic acid may be derived from one molecule of lactose. 
These reactions are represented by the following empirical 
equations :— 
(1) CoH 22011 ao 2 COs H1206 
(lactose) (glucose and galactose) 
(2) 2 Og H 1206 = 4 CsHgO3 
(hexose) (lactic acid) 
Milk is approximately a 5% or 0°15 N solution of lactose. 
If the increase of the acidity of milk during souring be due 
to the formation of lactic acid, then each extra cubic 
centimetre of N/10 alkali required to neutralise 100 cc. of 
milk after the onset of souring will correspond to the split- 
ting up of 0°0001 x 100/(0°015 x 4) = 0°17Z of the lactose 
present. The value of the depression of the freezing point 
for milk (—0°55° OC.) shows it to contain a number of 
dissolved particles equal to that in a 0°3 N solution. We 
have just seen, however, that milk is a 0°15 N solution of 
