1895 - 96 .] Mr D. F. Harris on Chemistry of Milk. 
79 
becoming a visible effect only in the acid medium. Both CaCl 2 
and acetic acid (10 per cent.) are precipitants of pro-casein. 
CaCl 2 is not a precipitant of pure caseinogen. Although one can 
make “ curds and whey ” in neutral milk, still the lay public have 
recognised the fact that acidity hastens the clot’s appearance in 
that the rennet essences for culinary purposes are highly acid. 
Keeping in view the acidity of the preparations of rennet (upon 
which point Dr Ringer gives us scarcely enough information), I 
repeated his experiments with milk, diluted with water 1 in 4, 
which, as he alleges, does not clot with Martindale’s faintly acid 
rennet. When this has been added to milk the acidity is very 
slight; if some of this mixture be boiled, and then either have 
added to it 1 m. acetic acid or 2 or 3 m. CaCl 2 , a clot is obtained. 
Again, I always obtained a clot when using A.R. instead of M.R, 
with 1 in 4 milk. Both these rennets formed pro-casein, but it 
required either acetic acid or CaCl 2 to make the work of the faintly 
acid rennet manifest. 
Further, on this point, the following experiments tell their own 
tale, R.C. + N.R. = 0, whereas R.C. + A.R. = clot, where R.C. = 
Ringer’s pure caseinogen. Before the use of Ringer’s test the 
following results would have corroborated the statement that 
“ acidity favours while alkalinity inhibits clotting.' >, Milk + A.R. = 
a clot at once, but milk rendered distinctly alkaline by the addition 
of yQ-th of its volume of NH 4 .HO heated with M.R. gives no clot. 
If we boil some of this last and add 2 or 3 m. of CaCl 0 we get a 
clot, proving that alkalinity does not inhibit or antagonise the 
formation of pro-casein, but inhibits, without antagonising, the fall 
of casein. As far as my observations go, rennet is antagonised 
(? killed) in stage one by the presence only of concentrated acid added 
to one half the volume of the caseinogen-containing liquid ; or by 
being heated to 100° C. Pro-casein formed in dilute solutions ( e.g ., 
Ringer’s caseinogen or milk 1 in 4) may not be precipitated by 
CaCl 2 alone, thus — R.C. + N.R. = 0, boil this, and add CaCl 2 , still 
no clot, then add 1 in acetic acid when a clot appears. Again, 
acidity was the only differential factor in the following in both of 
which CaCl 2 was present — R.C. + CaCl 2 + N.R. = 0 whereas R.C. + 
CaCl 2 + A.R. = clot. Phosphoric acid, I find, forms a firmer clot 
than other acid used at the same dilution. 
