36 
principle responsible for the bluing of guaiacum is volatile. He was 
led to believe, therefore, that this particular species of cyanogen 
C cyanogene ), which is responsible for the bluing of guaiacum, whatever 
its nature, is absorbed by certain substances which in its ordinary 
state permit of this action, but that when exposed to certain tempera- 
tures it enters into new combinations and obeys other laws which 
do not permit of its coloring guaiacum. 
To anyone at all familiar with the oxidizing ferments it is evident 
how close Planche came to the discovery of this remarkable group of 
substances. As a matter of fact he seems to have been the first to 
encounter them, and when he proved that heat destroys the power 
of milk and certain plant juices to blue guaiacum, he discovered one 
of their most remarkable characteristics, and had the state of knowl- 
edge regarding oxidation and fermentation been at this time what it 
was in the time of Schoenbein and Traube, there is scarcely room for 
doubt that he, and perhaps Taddei also, would have recognized the 
true nature of these substances and the part which they play in 
oxidation processes. 
From the time of Planche to that of Schoenbein and Traube there 
appears to have been but little advance in our knowledge of the 
guaiacum reaction, except the discovery of various substances, 
organic and inorganic, which effect this change of color to a greater 
or less extent. Thus Kegimbau, ( 340, 341 ) in letters to Planche, 
described the bluing of pills containing guaiacum resin, mercuric 
chloride, and white soap, and advanced the view that this bluing was 
due to the liberation of chlorine from the mercuric chloride, a view 
which Planche pointed out as erroneous; and Lodibert ( 274 ) reported 
results with dentifrices similar to those observed by Boullay ( 73 ). 
Pelletier and Deville ( 318 ) also published a paper on guaiacum in 1S44, 
in which, however, there is nothing of any particular interest. 
The further elucidation of the guaiacum reaction and the final 
discovery of the oxidizing ferments awaited the discovery of ozone 
and the renewed interest in the general subject of fermentation 
awakened by the writings of Pasteur and Liebig. In this connection 
it will be recalled that ozone was first recognized as a definite sub- 
stance by Schoenbein ( 368> 369 ). In 1845 Schoenbein ( 37 °) showed 
that guaiacum is colored blue by ozone. The interest attaching to 
Schoenbein’s earlier observations on the bluing of guaiacum by 
ozone is such in this connection that I shall present the subject in j 
practically the author’s own words. In his “ Note on guaiacum resin 
he goes on to say that — 
It has long been known that guaiacum resin under certain conditions becomes 
blue and that chlorine has the power of producing this change of color. I have found 
that bromine and iodine produce the same change. In consequence of the close 
similarity which chlorine, bromine, and iodine exhibit to ozone, one would naturally 
