Makgill. — Nature's Efforts at Sanitation. 143 



ferments which pass through the filter, giving us a solution 

 containing their active principles, but free from the germs 

 themselves. These ferments are the solvent agents breaking 

 up organic matter. A simple illustration of this solvent pro- 

 cess is seen in the liquefaction of gelatine by many varieties of 

 bacteria, while the breaking-up of sugar into alcohol and 

 carbonic acid is a familiar example of the reducing process. 

 The action of these ferments on animal matter is to break it 

 up, and results in the formation of much simpler bodies, such 

 as ammoniacal salts and what are known as putrefactive 

 alkaloids or ptomaines, with which we are all too familiar. 

 These are compounds of carbon, hydrogen, and nitrogen, with 

 or without oxygen, and are far less complex than the animal 

 tissues. Most are poisonous to man, some extremely so, and 

 are frequently the cause of illness — from eating sausages, 

 pies, and other forms of albuminous food in which putre- 

 factive changes have been allowed to commence ; and it is 

 worth noting that they are not destroyed by heat, so that 

 cooking, while it may kill the bacteria which produce the 

 change, does not render decomposed meat harmless. These 

 ptomaines must not be confounded with toxins — the poisons 

 formed in the living body by disease-producing bacteria, such 

 as those of cholera and diphtheria. I shall refer to toxins 

 again in connection with protection against disease. 



Putrefaction may be compared to a process of digestion of 

 organic matter by the bacteria which feed on it. It is first 

 liquefied and. converted into ptomaines and ammoniacal bodies. 

 These in turn are split up into simple salts and also gaseous 

 bodies, many of which have unpleasant smells, such as sul- 

 phuretted hydrogen and other less well-known unpleasant com- 

 pounds. Finally, still simpler gases are evolved, such as hydro- 

 gen, carbonic acid, and marsh gas. It is only when reduced to 

 these simple bodies that plants can utilise the organic matter. 

 The gases, such as carbonic acid, can be directly absorbed by 

 the leaves. But the ammoniacal bodies have to be carried 

 yet a step further. This is known as the process of nitrifica- 

 tion, and the work is done by bacteria in the soil. The 

 soluble putrefactive products are now soaked up by the soil or 

 worked into it by earthworms and insects. The putrefactive 

 organisms diminish, being crowded out by the nitrifying ones, 

 before the whole of the organic matter is converted into the 

 elemental gases. There are two sets of these nitrifiers, the 

 first acting on the ammoniacal salts, reducing them to nitrites. 

 The second are oxidizers, and convert the nitrites into 

 nitrates, the form in which the plant-roots can absorb them 

 for its nourishment. The whole of the organic matter is now 

 gone ; the plant-roots have taken up the salts and the leaves 

 many of the gases. Other gases have escaped back to the 



