LECTURE VIII. 



AT first sight you might think it an almost impossible thing to 

 obtain any exact knowledge on the metabolism (the constructive 

 and destructive internal changes) of such minute organisms as 

 bacteria, especially as they are generally found in media of such 

 a complex chemical nature. However, although our knowledge 

 of the physiology of bacteria is far from complete, the researches 

 of bacteriologists have opened for us, by means of their ingenious 

 devices, delicate methods and untiring perseverance, the entry to 

 a store of interesting facts regarding the metabolism of these 

 organisms. 



By special methods it has been possible to grow and accumulate 

 sufficiently large masses of bacteria to permit of chemical analysis. 

 In this way it has been found that the same elements enter into 

 their composition as into other plants, viz. carbon, hydrogen, 

 oxygen, nitrogen, sulphur, phosphorus, potassium, magnesium, 

 and calcium. Thus we know that supplies of these elements are 

 needed for their growth. When a mass of bacteria is dried it 

 loses about 85 per cent, of its weight. This loss represents the 

 water of the vacuoles of the cells and that held inbibed in the 

 colloidal substances of the cell protoplasm and proteins. After 

 drying there remains over about 13 per cent, of the original 

 weight of protein and about i per cent, of fat. And if this total 

 residue is burnt so that all organic substances are oxidised to 

 form carbon dioxide and water, there is left behind a weight of 

 mineral ash which represents sometimes as much as i per cent, 

 of the total weight. These constituents are very similar to those 

 found in other organisms and their relations to one another are 

 in no way exceptional. 



The examination of the drop of water taken from contact with 

 the decaying seaweed showed you that there were many kinds of 

 bacteria present which differed from one another in external 

 form. It is possible that they also differ in their metabolism. 

 Furthermore, among those which resemble one another in form, 

 there well may be differences in metabolism. Evidently there is 



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