INTRODUCTION. 1 7 



its special work and, though it is possible that many of them 

 started with certain powers in common, it seems that through 

 the exercise of some of these common powers under special 

 conditions they have become gradually so differentiated 

 functionally, that, as amongst organisms more highly 

 developed, each is able to carry on its own work best at 

 those special stages of the putrefactive process at which it 

 is found. It might at first sight appear that all this can 

 have but little bearing on any practical work in which we 

 are engaged, or in which we take an interest, but on more 

 careful consideration it will be found that these putrefac- 

 tive organisms really keep up the circulation of matter, 

 utilizing the excretions of living beings and the carcases of 

 dead animals and plants, after breaking them down into 

 their simplest constituents, to supply those elements that 

 are necessary for the nutrition of plants, allowing them to 

 present themselves in their most assimilable forms, and in the 

 proportions most suitable for the nutrition of the growing, 

 highly organized vegetable protoplasm. Bacteria in fact serve 

 to transform inert organic matter into inorganic substances. 

 This transformation, or " Mineralization" in most cases, com- 

 mences only after protoplasm has lost its vitality, and most 

 micro-organisms are capable of attacking this dead protoplasm 

 only ; though, as we shall find later, a certain number of 

 bacteria have acquired the faculty of being able to attack 

 even living protoplasm. The processes of decomposition 

 may be divided into two kinds : first, those going on as the 

 result of the activity of organisms that are capable of taking 

 up their oxygen from the air, and, second, those the result 

 of the activity of organisms that so break up and rearrange 

 the organic molecules containing oxygen, that not only do 

 they, the bacteria, take up oxygen themselves but they allow 

 of its being handed on to the products to which in their 

 processes of metabolism they give rise. It is probable that 

 here we have to do, not only with nascent oxygen, but that 

 we have certain products set free during the process of 

 decomposition which seize upon oxygen with very great 

 avidity. This decomposition or rearrangement is spoken 

 of as a process of nitrification, or a conversion of the nitro- 

 genous elements into ammonia, nitrous and nitric acids, 

 carbonic acid and water, or, speaking more generally, it 

 may be said to be a process of mineralization of the organic 



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