346 
plants, which supposes these substances to originate from formic 
aldehj^de, which in turn results from the reaction of carbon dioxide 
with water, as indicated in the following equations: 
(1) C0,+2H,0=^5^5g^+HA 
and 
In this reaction hydrogen peroxide is formed, and of course if 
allowed to accumulate in the cell would soon j^ut a stop to all of its 
vital activities. According to Loew, however, the accumulation of 
hydrogen peroxide would be prevented by its decomposition, prac- 
tically as fast as formed, by the catalase. 
Quite recently Usher and Priestley (30) have succeeded in demon- 
strating the decomposition of carbon dioxide by means of chlorophyll 
within and outside of the plant and in proving the presence of formic 
acid, formic aldehyde, and hydrogen peroxide among the products 
of the decomposition. As already stated this work is of unusual 
interest, and if confirmed by subsequent investigations will go a long 
way toward bringing about an understanding of this important 
biochemical process and will enable us to understand better the 
function of the catalases in general. Of course if hydrogen peroxide 
or similar peroxides are present in milk they must of necessity have 
a different origin from the peroxides occurring in the cells of chloro- 
ph}dlous plants. It is readily conceivable, however, that such 
peroxides might originate in animal tissues and secretions in other 
ways, and if, in the one case, it is established that the function of 
catalase is to destroy hydrogen peroxide and thereby prevent its 
accumulation in the cell, it will probably turn out that it has a 
similar function in milk or in whatsoever associations it is found. 
It is held by some that the catalases are not oxygen carriers, and in 
this connection Lesser (31) has shown that the decomposition of 
hydrogen peroxide by catalase does not lead to the oxidation of fat 
or carbohydrates. According to this author catalase is to be regarded 
as a substance capable of taking up oxygen and of giving it up again 
under certain circumstances. 
Peroxidases . — The idea that milk contains true oxidizing ferments 
or oxidases proper probably originated from the fact that in the 
earlier work on this subject old tinctures of guaiacum were employed 
in making the tests. As is well known, old tinctures of guaiacum 
frequently exhibit reactions which are not shown by the perfectl}^ 
fresh tinctures (32). This has been accounted for on the supposition 
that on standing exposed to air and light substances of the nature 
