3 o8 



THE POPULAR SCIENCE MONTHLY 



influence of light is at the basis of many photographic processes, 

 notably those of making blue prints and tintypes. Now, of 

 course, in the meteorites containing native iron, not all of the 

 iron is oxidized, and the iron is contained in its less oxidized con- 

 dition in the other associated minerals, such as the yellowish- 

 green mineral known as chrysolite, sometimes used for a gem. In 



\& 



^ 



Ocean 



Ocean 



Fig. 1. Much-exaggerated Sections of the Earth through the Equator illustrating 

 ((/) the tidal effect on a rigid earth with a fluid envelope, (A) on a yielding earth. 



general, also (there are exceptions), the rocks which contain less 

 silica and more iron have their iron less oxidized. By analogy, 

 as we go from the oxidizing effect of the atmosphere toward the 

 center of the earth, we may expect finally to encounter rocks not 

 oxidized even in the less degree. To sum the argument up in a 

 nutshell, we find among the rocks furnished us by volcanic and 

 igneous agencies from various depths in the earth a series from 

 quartzose and feldspathic rocks to those with less quartz ami 

 feldspar, more iron, less oxygen, and greater weight, in which the 

 presence of a trace of nickel and the occasional occurrence of 

 diamonds and native iron betoken a kinship to the meteorites. 

 The latter in every way continue this series toward a goal which 

 is nearly pure iron, and the weight of the earth as a whole is 

 consistent with this idea that it is largely iron, almost purely so at 

 the center, but gradually, perhaps not perfectly uniformly, grow- 

 ing more quartzose toward the crust. 



One question still remains to us : In what condition is the in- 

 terior of the earth ? Is it a molten fluid or what ? If we look at 

 the downward increase in temperature alone it would seem as if 

 within thirty miles a heat would be reached where even pure 

 iron, which is much less fusible than cast iron containing carbon, 

 would be quite fluid. If the earth were freely fluid, however, it 



