88 BACTERIA. 



a fermenting fluid being found to rise without the addition 

 of any external heat. An object lesson may be used to 

 illustrate the processes of breaking down and re-combination 

 that go on in fermentation. Let us suppose that we have 

 a tall tower built up of two kinds of blocks, placed one on the 

 top of another.; first a large block and then three small ones, 

 then another large block and then three more small ones ; 

 let us consider that there are pulling on these blocks elastic 

 bands of different strengths, one very strong one between the 

 two large blocks, another very strong one between each pair 

 of the small blocks, and two sets of three weaker bands run- 

 ning from the larger blocks to the smaller ones ; these sets 

 of blocks are so arranged that if left perfectly undisturbed 

 they will remain piled up one on another forming a tower of 

 considerable height. The higher the tower the more easily, 

 as a rule, will it be upset, though this depends on the way in 

 which the larger and smaller blocks are distributed in the 

 structure. If, now, some disturbing element comes in, and 

 if one of the blocks is withdrawn or is even slightly shaken, 

 the whole structure may collapse, even the movement 

 of a small top block may bring this about by altering the 

 tension on one of the elastic bands, so disturbing the 

 equilibrium of tension on the whole tower, that the two 

 large blocks, with their strong elastic band, are set free 

 and fall to the ground ; the pairs of smaller blocks, with 

 their strong uniting bands, also fall to the ground, and the 

 other thin bands of which we have before spoken are now so 

 far stretched or broken that their influence in holding the 

 different sets of blocks together may now be left altogether 

 out of account. But in falling from the top of the tower to 

 the bottom energy of position has been lost, and in falling 

 and striking the ground a block may do a certain amount of 

 work ; if it were in connection with a series of pulleys it 

 might be made to lift a certain weight ; if it were to strike 

 the ground with sufficient force it might produce a flash of 

 light, or a certain amount of heat. We are, perhaps, not 

 justified in stating that this is exactly analogous to what 

 takes place in fermentation, but it is sufficiently so to 

 explain the nature of the process, if we consider that the 

 kinds of blocks that are built into our tower are multiplied. 

 In the most characteristic and best form (for our purposes) of 

 fermentation th^t can be taken a.s an example, we have 



