CH. xxv.] FERMENTS. 403 



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 intei mediate angles 

 there will be intermediate degrees of illumination. 



If the analyser at.d 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 substance is dextro- or levo-rotatory. The amount 

 of rotation is measured by the number of angles through which the anaylser 

 has to be turned in order to obtain the full illumination. This will vary 

 with the length of the tube and (he strength of the solution. 



Ferments. 



The won! 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 ' fermenta- 

 tion.' The agent, yeast, which produces this, 

 is called the ferment. Microscopic investiga- 

 tion 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 decompos- 

 ing urine, and the formation of vinegar (acetic *">& 338. Cells of the 



yeast plant in innnnai 



acid) from alcohol are brought about by of budding, 

 very similar organisms. The complex scries 



of changes known 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 temperature or by certain substances (carbolic 

 acid, mercuric chloride, <fec.) called antiseptics. 



The 'germ theory' of disease explains the infectious diseases 

 by considering that the change in the system is of the nature of 

 fermentation, 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 bacteria appear to be either alkaloidal 

 (ptomaines) or proteid in nature. The existence of poisonous 

 proteids is a very remarkable thing, as no chemical differences 

 can be shown to exist between them and those which are not 



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