90 BACTERIA. 



the rearrangement that takes place when the smaller towers 

 are formed. It may be asked, How does the tilting over of 

 the bricks bear on all this ? The first tap that you give 

 them may be said to represent the action of some ferment ; 

 the tap that your friend gives at the same time may be held 

 to represent one of the conditions essential for the production 

 of a ferment, say the presence of a certain amount of light ; 

 whilst the tap that a second friend gives may be looked upon 

 as representing a certain degree of temperature ; it may be 

 a forcible tap, representing 40 C., or it may be a slighter one, 

 representing a temperature of 20 C. only, in which case you 

 do not get sufficient energy out of the ferment to start the 

 process. One or two only of your taps may upset certain 

 towers or certain lines of bricks, but other towers that have a 

 somewhat broader basis or a more definite arrangement 

 require the whole three forces to be applied, and these have 

 to be applied in certain definite proportions if the same 

 results are to be obtained in every case. Thus, for example, 

 by graduating the power of the disturbing force, the equili- 

 brium of your tower may be simply upset and the elastic 

 bands drawing in certain directions cause a rearrangement 

 of the blocks according to the strength with which these 

 bands pull ; if, however, instead of simply just distributing 

 the equilibrium to allow of the new arrangements taking 

 place, your tower is struck with a sledge hammer, or over- 

 turned by a pistol shot, or by the explosion of a charge of 

 dynamite near it, the conditions are so altered that you 

 cannot rely upon any plan of rearrangement being adhered 

 to. As a result you simply obtain a series of disconnected 

 blocks or of much smaller and irregular towers. It will be 

 said that this regular breaking up is exactly what takes place 

 in the breaking up of chemical combinations. This is perfectly 

 true, for that is exactly what fermentation is. It is the upset- 

 ting of the equilibrium of unstable compounds by most deli- 

 cately adjusted forces which are so accurate, so constant, and 

 so delicate, that the stronger affinities of certain elements for 

 one another are allowed to act to their full extent, and regular 

 stable combinations are formed always in a definite manner. 

 Let us take first such a substance as nitrogen trichloride or 

 nitrogen teriodide. If a sharp blow be given to a small quantity 

 of either of these materials, or if either be heated to a certain 

 temperature, it will immediately break down into nitrogen 



