CHEMICAL CHANGES 155 



free in the soil. In other eases they live in the root nodules 

 of leguminous plants. 



The Carbon Cycle.— All plants and animals are con- 

 stantly producing carbon dioxide during the process of 

 metabolism. A part, at least, of the growth energy of every 

 living cell is secured by the oxidation of carbon compounds, 

 and carbon dioxide is eliminated. However, there are a few 

 microorganisms and the whole group of green plants which 

 are capable of synthesizing carbon dioxide and water into 

 complex organic compounds, particularly carbohydrates 

 and proteins (together with ammonia). We have constantly,- 

 therefore, in action the two processes, formation of carbon 

 dioxide from complex organic compounds, a process of 

 oxidation, and the formation of organic compounds from 

 carbon dioxide, a process of reduction. Microorganisms are 

 constantly producing considerable quantities of carbon 

 dioxide in the decomposition of organic material. A few, 

 however, have been noted, such as those which oxidize 

 ammonia to nitrites, which apparently can make use of 

 carbon dioxide in the synthesis of their own food. The 

 carbon cycle is therefore comparatively simple, consisting of 

 this alternate oxidation and reduction. 



The Sulphur Cycle. — Hydrogen sulphide apparently 

 originates in nature in two ways : first, as the result of the 

 decomposition of organic compounds containing sulphur by 

 microorganisms with the formation of free H2S ; and second, 

 the reduction of sulphates under anaerobic conditions by 

 microorganisms with the formation of free HgS. Hydrogen 

 sulphide is also found in water of certain springs. 



The ordinary putrefactive and decay-producing bacteria, 

 for the most part, are capable of liberating hydrogen 

 sulphide from the complex protein compounds in which it 

 is found. A much smaller number of species are capable 

 of changing sulphates to sulphides under suitable condi- 

 tions. In some cases the decomposition of sulphates to 



