THE DIGESTIVE JUICES 63 



reaction) is produced. This should be carefully compared with the violet 

 tint given by unaltered albumin. 



(c) To a third portion of the fluid in test-tube B add a drop of nitric acid 

 proteoses or propeptones are precipitated. This precipitate dissolves on 

 heating and reappears on cooling. 



5. Repeat these three tests with the digested white of egg in test-tube D. 



6. Examine an artificial gastric digestion which has been kept a week. 

 Note the absence of putrefactive odour ; in this it contrasts very forcibly 

 with an artificial pancreatic digestion under similar circumstances. 



FERMENTS 



The word fermentation was first applied to the change of sugar 

 into alcohol and carbonic acid by means of yeaat. 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 



D i ' t FIG. 12. Cells of the yeast 



urea into ammonium carbonate in decomposing ptat in process of bad- 



. CUHffi between which 



urine, and the formation of vinegar (acetic acid) are some bacteria. 



, 111 -, -, i o , (Yeo's 'Physiology.') 



irom alcohol are produced by the growth 01 very 

 similar organisms. The complex series of changes known as putre- 

 faction, which are accompanied by the formation of malodorous 

 gases, and which are produced by the growth of 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, 

 &c.) called antiseptics. The organisms produce fermentative effects 

 by shedding out soluble ferments or enzymes. 



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 

 bacteria appear to be either alkaloidal (ptomaines) or protein in 

 nature. The existence of poisonous proteins is a very remarkable 

 thing, as no profound chemical differences have yet been shown to 

 exist between them and those which are not poisonous, but which are 



