441 
was added with a pipet, the contents of the test-tube quickly rever- 
sed several times and again put into the waterbath. By carefully 
moving the test-tube from time to time, the moment when the milk 
became thicker, could be observed. When this change began, curd- 
ling was very near. The test-tube then was taken out of the water- 
bath and carefully inclined, so that some of the contents slowly 
moved along its walls, till at a cerfain moment floccules of about 
‘/,m.m. suddenly appeared in the milk which adhered at its walls. 
This point was taken as the curdling-point. As milk-eurdling is 
delayed by shaking and by cooling, care was taken to avoid all 
unnecessary movement and cooling. With some practise it is easy 
to reduce both factors to a minimum and then the eurdling-time can 
be accurately determined. In the case of milk without citrate, the 
curdling-time seldom varied more than 15 seconds (in a curdling- 
time of 2'/, minute); usually the observations differed less. After 
adding salts which only give a small delay, analogous differences were 
obtained ; in the case of strongly delaying salts, the differences were 
somewhat larger, but always agreed sufficiently. Every curdling-time 
was determined in duplo or in triplo and the exact values found 
by taking the average. Curdling-times of more than 2 hours cannot 
be trusted because of the possibility of bacterial action. 
It was found, that the kind of milk investigated on subsequent 
days with the same solution of rennet (1:17) gave curdling-times 
which varied little. In order to make observations on different days 
as well comparable as possible, the solution of rennet was always 
taken somewhat more or less diluted till a curdling-time of 2 minutes 
18 seconds exactly was obtained, this being the value on the first day. 
The lengthening of curdling-time found when the milk contained 
the quantities of citrate given below, is seen from the following 
figures : 
0.00016 N delay 17 seconds | 0.0020 N delay … 289 seconds 
0.00032 N 5 dd | 0.0030 N 5 27 min.(27’) 
0.0008 N y 105 ,, |0.0040N 4 Bl or 
0.0016 N i 191 ‘ft 0.0080 N De 9 hours. 
These figures give the following curve (see p. 442). 
Which is the best concentration to compare citrates with the 
substituted products? When the concentration is sufficiently large 
all salts inhibit milk-eurdling. The characteristic of citrate-action is 
the fact that curdling is prevented in concentrations in which other 
salts give a scarcely perceptible delay. In general therefore the 
results of the comparative investigation will be the more correct, 
