3 2 2 CHEMISTR Y OF THE DIGESTIVE PR CESSES. 
in glass was due to accumulation in the solution of digested products. Similar 
experiments on the digestion of various forms of proteid, by pepsin and hydro- 
chloric acid, dialysing into hydrochloric acid of equal concentration, have been 
made by Chittenden and Amerman, 1 who found that removal of the products 
of digestion did not essentially favour peptonization or alter the relative 
amount of albumose and peptone formed. 
('once nt ration of enzyme. — The rapidity with which zymolysis takes 
place naturally varies with the concentration of the enzyme in the 
solution, as well as with the concentration of the material to be digested, 
when this is soluble. Roberts found in the case of conversion of starch 
by the diastatic enzyme of the pancreas, that the amount of standard 
starch mucilage which can be converted in a given time and at a given 
temperature varies directly as the quantity of active solution employed. 
Schiitz 2 found in the digestion of proteid by pepsin, that when the 
solutions employed were sufficiently dilute, the amount of conversion 
was proportional to the square roots of the quantities of pepsin present. 
Any such rule can only bold within certain limits of concentration, a 
maximum being reached beyond which further concentration of the 
enzyme has no effect. 
Methods of estimating the relative activity of digestive solu- 
tions. — As none of the enzymes have been isolated in a pure condition, 
it follows that there is no means of estimating the absolute amount of an 
enzyme in solution. This is practically never a matter of any moment, 
but a problem which often presents itself in practical work on digestion 
is that of estimating the relative activities of two digestive extracts. 
The activity of a diastatic enzyme can be most accurately estimated 
by determining the amount of sugar (maltose) formed under given con- 
ditions in a given time by a given volume of the solution, acting on a 
measured volume of a standard solution of starch mucilage ; this, however, 
is a tedious and troublesome process, and for most purposes a sufficiently 
accurate process is that of observing when the starch has all disappeared, 
as shown by the failure of the iodine reaction. 
Such a method has been introduced by Roberts. 3 He varies the amount 
of the diastatic solution added until the " achromia point " is reached within a 
period lying between the limits of four and six minutes. This achromic point 
is that point at which the starch solution ceases to give a yellow tinge with 
iodine, when accordingly the solution contains only achroodextrins and maltose. 
Roberts defines the diastatic value of a solution (denoted by the symbol D) 
by the volume in cubic centimetres of a standard starch mucilage which can 
l:>e converted to the achromic point by 1 c.c. of that solution, acting during five 
minutes at a temperature of 40° C. 
The standard solution of starch mucilage must be prepared fresh ; it is 
made by stirring up 5 grms. of pure potato starch with 30 c.c. of water, and 
pouring slowly into nearly 470 c.c. of water, which is kept boiling. The 
mixture is stirred and boiled for a few seconds, and finally accurately made up 
to 500 c.c, thus giving a standard solution (1 per cent.) of starch. 
The solution of iodine used is made by diluting 1 part of the liq. iodi of 
the Pharm. Brit, with 200 parts of water. 
In making a determination, one proceeds as follows : — Ten c.c. of the 
standard starch mucilage are diluted with distilled water to 100 c.c. and 
1 Joum. Physiol., Cambridge and London, 1S93, vol. xiv. p. 483. 
2 Ztschr. f. physiol. CJiem., Strassburg, 1885, Bd. ix. S. f>77. 
3 Diastasimetiy, In " Digestion and Diet," London, 1891, p. 68. 
