CH. XXV.] FERMENTATION 405 



arrows are shot vertically they will puss easily through the gaps between the 

 palings, but if they are shot horizontally they will be unable to pass through at 

 all. This rough illustration will help us in understanding what is meant by polarised 

 light. Ordinary light is produced by the undulations of aether occurring in all 

 directions at right angles to the path of propagation of the wave. Polarised light 

 is produced by undulations in one plane only ; we may compare it to our flat 

 arrows. 



In a polarimeter, there is at one end of the instrument a Nicol's prism, which 

 is made of Iceland spar. This polarises the light which passes through it ; it is 

 called the polariser. At the othr end of the instrument is another called the 

 analyser. Between the two is a tube which can be filled with fluid. If the analyser 

 is parallel to the polariser the light will pass through to the eye of the observer. 

 But if the analyser is at right angles to the polariser it is like the flat arrows hitting 

 horizontally the vertical palings of the fence, and there is darkness. At inter- 

 mediate angles there will be intermediate degrees of illumination. 



If the analyser and polariser are parallel and the intermediate tube filled with 

 water, the light will pass as usual, because water has no action on the plane of 

 polarised light. But if the water contains sugar or some " optically active " substance 

 in solution the plane is twisted in one direction or the other according as the sub- 

 stance is dextro- or levo-rotatory. The amount of rotation is measured by the 

 number of angles through which the analyser has to be turned in order to obtain 

 the full illumination. This will vary with the length of the tube and the strength 

 of the solution. 



Ferments. 



The word fermentation was first applied to the change of sugar 

 into alcohol and carbonic acid by means of yeast. The evolution of 

 carbonic acid causes frothing and bubbling ; hence 

 the term " fermentation." The agent, yeast, which 

 produces this, is called the ferment. Microscopic 

 investigation shows that yeast is composed of 

 minute rapidly-growing unicellular organisms 

 (torulse) belonging to the fungus group of plants. 



The souring of milk, the transformation of 

 urea into ammonium carbonate in decomposing 

 urine, and the formation of vinegar (acetic acid) 

 from alcohol are brought about by very similar FlG - 342. ceiis of the 



PIT yeast plant in process 



organisms. 1 ne complex series 01 changes known of budding. 

 as putrefaction, which are accompanied by the 

 formation of malodorous gases, and which are produced by the 

 various forms of bacteria, also come into the same category. 



That the change or fermentation is produced by these organisms 

 is shown by the fact that it occurs only when the organisms are 

 present, and stops when they are removed or killed by a high tem- 

 perature or by certain substances (carbolic acid, mercuric chloride, 

 etc.) called antiseptics. 



The "germ theory" of disease explains the infectious diseases by 

 considering that the change in the system is of the nature of fermen- 

 tation, and, like the others we have mentioned, produced by microbes ; 

 the transference of the bacteria or their spores from one person to 

 another constitutes infection. The poisons produced by the growing 



