INTRODUCTION 9 



as that of the same element in the protoplasmic molecule of 

 the vegetable. 



The changes to which we have referred are concerned 

 only with the relationships of bacteria and similar micro- 

 organisms to the protoplasmic molecule in the economy of 

 nature. Sulphur may of course be removed from the bowels 

 of the earth, and may be taken up as a sulphate or a sulphide 

 into the cycle governed by the activities of microorganisms. 

 Or again, the sulphur may lie dormant in such substances as 

 coal, and, after a rest of some millions of years in that form, 

 be once more taken up into the stream of that cycle. When 

 coal is burnt sulphur compounds are liberated into the air, 

 or spread over the surface of the earth. The converse must 

 also follow^, namely, the removal from the cycle of sulphur 

 compounds which may lie dormant for seons before being 

 brought back into circulation. Speaking generally, all 

 microorganismal and all combustion processes either directly 

 or indirectly bring about the complete oxidation of the 

 of the sulphur atom, and the reactions which bring about 

 this condition are exothermic. 



The sulphur bacteria are thus like all plants and animals 

 in that they require sources of energy to make possible the 

 various manifestations associated with life. These manifesta- 

 tions are necessarily in their totality exothermic processes. 

 As will be shown in the following pages, however, it is probable 

 that the purple sulphur bacteria are able to do constructive 

 work in virtue of their ability to absorb energy from the sun's 

 rays. In this they resemble the plants and animals that pos- 

 sess chlorophyll, and as in the case of green plants such energy 

 is directed to the building up from simpler compounds of more 

 complex substances which thus become storehouses of potential 

 energy. The conversion of stored energy to what may be named 

 " vital " energy takes place when the more complex substances 

 revert to simpler forms, thereby liberating some of their energy. 

 The same laws regulate the changes of energy in the animate 

 as in the inanimate world. 



