358 



THE WORLD OF LIFE 



CHAP. 



respectively of the earth -mass. Then follow calcium, 

 magnesium, sodium, and potassium, contributing from 

 about 4 to 2 per cent of the whole ; while no other element 

 forms so much as one per cent, and the majority probably 

 not more than one-fiftieth or one-hundredth of one per cent. 

 The gases, hydrogen and nitrogen, are, however, 

 exceedingly important as forming with oxygen the atmo- 

 sphere and the oceans of the globe, which by their purely 

 physical action on climate, and in causing perpetual changes 

 on the earth's surface, have rendered the development of the 

 organic world possible. These ten elements appear to be all 

 that were necessary to constitute the earth as a planet, and 

 to bring about its varied surface of mountain and valley, 

 rivers and seas, volcanoes and glaciers ; but in order to 

 develop life, and thus clothe the earth with ever-growing 

 richness of vegetation and ever-changing forms of animals to 

 be sustained by that vegetation, four other elements were 

 required — carbon, sulphur, phosphorus, and chlorine — but 

 these being either gaseous or of very small specific gravity, 

 and thus existing (perhaps exclusively) near the earth's 

 surface, comparatively little of them was needed. 



List of the More Important Elements 



forming a large proportion of the earth's substance, are not essential 

 constituents of protoplasm, although occasionally forming part of it. 



In the list of the more important elements here given, I 

 have arranged them in two series, the first showing the 



