PRODUCTS OF VITAL ACTIVITY. 135 
The well-known Bacillus prodigiosus (also described as a micro- 
coccus) produces a red pigment which is insoluble in water but solu- 
ble in alcohol. By the addition of an acid the color becomes car- 
mine and then violet, which is changed to yellow by an alkali. The 
color is said by Schottelius to be diffused in the young cells, and 
after the death of the cells to be present in their vicinity in the form 
of granules. The same author has shown that by subjecting the 
bacillus to special conditions a variety may be obtained which no 
longer produces pigment. 
The conditions which govern the formation of pigment in the 
chromogenic bacteria are determined with comparative facility be- 
cause the results of changed conditions are apparent to the eye; in 
the case of products which are not colored the difficulties attending 
the study of these conditions are much greater, but the results are in 
many instances more important. The following are among the best 
known pigment-producing (chromogenic) bacteria : 
Staphylococcus pyogenes aureus, Staphylococcus pyogenes cit- 
reus, Sarcina aurantiaca, Sarcina lutea, Bacillus cyanogenus, Bacillus 
janthinus, Bacillus fluorescens liquefaciens, Bacillus indicus, Bacillus 
pyocyaneus, Bacillus prodigiosus, Spirillum rubrum. 
Liquefaction of Gelatin.—Many species of bacteria, when 
planted in a medium containing gelatin, cause a liquefaction of the 
gelatin in the immediate vicinity of the growing microérganisms, 
while many others multiply abundantly in the same medium with- 
out liquefying the gelatin. This character, as first shown by Koch, 
is an important one in the differential diagnosis of species which re- 
semble each other in form and in other respects. It has no relation 
to pathogenic power, as some liquefying organisms are harmless sap- 
rophytes and some deadly disease germs, while, on the other hand, 
non-liquefying bacteria may be very pathogenic or quite innocent. 
Liquefaction is produced by a soluble peptonizing ferment formed 
during the growth of the cells. This is shown by the fact that if a 
liquefying organism is cultivated in bouillon and the living cells re- 
moved by filtration or killed by heat, the power of liquefying gelatin 
remains in the culture fluid. This was first observed by Bitter (1886) 
and independently by the writer in 1887. In experiments made to 
determine the thermal death-point of various bacteria the writer 
found that when cultures of liquefying species were subjected to a 
temperature which killed the microdrganisms, a few drops of the 
culture added to nutrient gelatin which had been liquefied by heat 
prevented it from subsequently forming a solid jelly when cold. 
In a study of the ferments produced by bacteria which cause 
liquefaction of gelatin—“ tryptic enzymes”—made by Fermi, in the 
