VITAL PHENOMENA OF BACTERIA. 59 
Some chemical substances seem to exert a peculiar 
attraction for bacteria, known as positive chemotazis, 
while others repel them—negative chemotawis. More- 
over, all varieties are not affected alike, for the same 
substances may exert on some bacteria an attraction 
and on others a repulsion. Oxygen, for example, 
attracts aérobic and repels anaérobic bacteria, and for 
each variety there is a definite proportion of oxygen, 
which most strongly attracts. The chemotaxic prop- 
erties of substances are tested by pushing the open end 
of a fine capillary tube, filled with the substance to be 
tested, into the edge of a drop of culture fluid contain- 
ing bacteria and examining the hanging drop under 
the microscope. Weare able thus to watch the action of 
the bacteria and note whether they crowd about the tube 
opening or are repelled from it. Substances showing 
positive chemotaxis for nearly all bacteria are peptone, 
urea, and very weak solutions of bichloride of mercury. 
While among those showing negative chemotaxis are 
aleohol and many of the metallic salts. 
The Production of Light. Bacteria which have the 
property of emitting light are quite widely distributed 
in nature, particularly in media rich in salt, as in sea- 
water, salt fish, etc. Many of these, chiefly bacilli and 
spirilla, have been accurately studied. The emission of 
light is a property of the living protoplasm of the bac- 
teria, and is not usually due to the oxidation of any pho- 
togenic substance given off by them; at least only in two 
instances has such substance been claimed to have been 
isolated. Every agent which is injurious to the exist- 
ence of the bacteria affects this property. Thus, cold 
paralyzes them and interrupts their power of emit- 
ting light. - High temperature, acids, chloroform, etc., 
