_Bacillus Pyocyaneus._--Distinctly green or blue in tint. _Bacillus Tuberculosis._--Thick, curdy, white paste, or thin, greenish, with small, cheesy lumps or even with bone spicules. _Actinomycis._--Thick, brownish white, with small, firm, gritty or chalky nodules of yellow color. _Ameba Coli._--Thick, brownish red. BACTERIAL DETERMINATION AS AN INDICATION IN TREATMENT. There is a practical side of great importance pertaining to the recognition of the nature of the infectious organism in many cases of suppuration and abscess. For instance, pus which is due to streptococcus invasion indicates a collection which should be freely evacuated and carefully drained. This is also true in essential respects of staphylococcus pus, particularly that due to the streptococcus aureus. Putrid pus from any source requires disinfection and free drainage, the former preferably perhaps by hydrogen dioxide. Pus which is due to the colon bacillus is not often extremely virulent, which accounts for so many cases of appendicitis recovering with or without operation. A collection of this pus needs little more than mere drainage and opportunity for escape. Pus from a recognizable tuberculous source may still contain living tubercle bacilli. This means either that the cavity whence it came should be completely destroyed and eradicated, or else that the margins of the incision or opening through which it has escaped should be so cauterized that infection of a fresh surface is impossible. The same is true of abscesses due to glanders bacilli and to certain cases of suppurating bubo following chancroid, where the whole course of events shows the virulent character of the organisms at fault. SUPPURATION. Although it may be possible to produce in certain laboratory experiments metamorphosed material which very closely simulates pus, or, in fact, by injection of chemical irritants, to sometimes imitate the suppurative processes, nevertheless, the student should be brought face to face with the statement, to which for surgical purposes there is no practical exception, that suppuration, _i. e._, _formation of pus, is due to the presence in the tissues of the specific irritants already catalogued and described, and of the peculiar peptonizing or other biochemical changes which bacteria exert upon living animal cells_.