IV.] BACTERIA. 25 



spirally coiled body. No such structure, however, can be 

 made out in the straight Bacteria, and it remains doubtful 

 whether they possess cilia which are too fine to be rendered 

 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 de- 

 lusion. 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 contraction. 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 (without 

 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 Torulce ; 

 arid 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 



