77 
color for seventeen days. Even egg albumin and blood serum were 
found to retard the action of the ferment. 
The changes of color produced by the action of tyrosinase on 
tyrosin have been carefully studied by Gessard ( 183 ) . According to 
this author, when tyrosinase is added to an aqueous solution of 
tyrosin, oxygen is absorbed and the liquid takes on a rose color which 
gradually becomes reddish yellow, rapidly changing to mahogany 
red and then to garnet. He also observed the most marked colora- 
tion in the upper layers of the liquid, due to immediate contact with 
the air. Continuing these studies, Gessard ( 189 ) observed the colora- 
tion of tyrosin by tyrosinase to consist of two distinct phases, the 
first of which alone, viz, the change to rose and then to red, is, accord- 
ing to this author, attributable to the ferment. After a certain 
interval, depending on general conditions, the solutions acquire a 
violet color, and finally yield a black precipitate (melanine), leaving 
the supernatant fluid perfectly colorless. When, for example, the 
red liquid resulting from the action of tyrosinase upon tyrosin is 
exposed to a vacuum, it slowly becomes colorless. Now, on expo- 
sure to the air, the decolorized solution acquires a violet instead of a 
red color. Hence, a new substance has been produced by the 
reduction of the first substance in the vacuum, from which the first 
substance apparently can not be regenerated. The substance thus 
obtained by exposing the reddened solutions of tyrosin to a vacuum 
has been found to be very oxidizable and is characterized by its 
yielding a violet solution on oxidation, the coloring matter showing 
a great tendency to separate from the solution in the form of a black 
precipitate. The production of the violet compound as the result 
of the action of tyrosinase on tyrosin is also facilitated by the presence 
of certain salts, a fact which would probably explain the production 
of the black .compound under natural conditions. 
Aside from the fact that melanines are formed by the oxidation of 
tyrosin through the agency of tyrosinase, but little is known as to the 
precise mechanism of the process. According to Gonnermann ( 195 ), 
homogentisic acid, hydroquinone-acetic acid — 
/(OH), 
C 6 H 3 
\ch 2 cooh 
I'! 
is the principal product resulting from the action of tyrosinase on 
tyrosin in the presence of oxygen. According to this author, tyro- 
sinase is not an oxidizing but a hydrolytic ferment, and his conception 
of the process is that the homogentisic acid produced by the action of 
tyrosinase on tyrosin in the presence of air results from the spon- 
taneous oxidation of an unknown product of the hydrolysis of tyrosin 
by tyrosinase and not from the direct oxidation of the tyrosin, through 
