26 ELEMENTARY BIOLOGY. [CHAP. 



visible by our microscopes, or whether their movements are 

 due to some other cause. Many forms, such as the Vibriones, 

 so common in putrefying matters, appear obviously to have 

 a wriggling or serpentiform motion, but this is an optical 

 illusion. In this Bacterium, as in all others, the body does 

 not rapidly change its form ; but its joints are bent zig-zag- 

 wise, and the rotation of the zig-zag upon its axis, as it 

 moves, gives rise to the appearance of undulatory contrac- 

 tion. A cork-screw turned round, while its point rests 

 against the finger, gives rise to just the same appearance. 



Bacteria, in the still state, very often become surrounded 

 by a gelatinous matter, which seems to be thrown out by 

 their protoplasmic bodies, and to answer to the cell-wall of 

 the resting Protococcus. This is termed the Zoogloea form of 

 Bacterium. 



Bacteria grow and multiply in Pasteur's solution (with- 

 out sugar) with extreme rapidity, and, as they increase in 

 number, they render the fluid milky and opaque. Their 

 vital actions are arrested at the freezing point. They thrive 

 best in a temperature of about 30 C. but, in most fluids, 

 they are killed by a temperature of 60 C. (140 F.). 



In all these respects Bacteria closely resemble Tdrulce ; 

 and a further point of resemblance lies in the circumstance 

 that many of them excite specific fermentative changes in 

 substances contained in the fluid in which they live, just as 

 yeast excites such changes in sugar. 



All the forms of putrefaction which are undergone by 

 animal and vegetable matters are fermentations set up by 

 Bacteria of different kinds. Organic matters freely exposed 

 to the air are, in themselves, nowise unstable bodies, and, 

 if due precautions have been taken to exclude Bacteria, 

 they do not putrefy, so that, as has been well remarked, 

 " putrefaction is a concomitant not of death, but of life." 



