222 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



more clearly some points in that of the metals themselves. I 

 speak of phosphorus and iodine. 



You all know the essential part which the former of these, 

 combined as phosphate of lime, plays in the animal economy, 

 in the formation of bones ; and how plants require for their 

 proper growth and development a certain amount of phos- 

 phorus. Ordinary soils contain only a few thousandths of this 

 element, yet there are agencies at work in nature which gather 

 this diffused phosphorus together in beds of mineral phosphates 

 and in veins of crystalline apatite, which are now sought to 

 enrich impoverished soils. Iodine, an element of great value 

 in medicine and in the art of photography, is widely distributed, 

 but still rarer than phosphorus ; yet it abounds in certain min- 

 eral waters, and is, moreover, accumulated in marine plants. 

 These extract it from the waters of the sea, where iodine exists 

 in such minute quantities as almost to elude our chemical 

 tests. (See the Appendix, page 237.) 



There are probably no perfect separations in nature. We 

 cannot, without great precautions, get any chemical element 

 in a state of absolute purity, and we have reason to believe 

 that even the rarest elements are everywhere diffused in infini- 

 tesimal quantities. The spectroscope, which we have lately 

 learned to apply to the investigation alike of the chemistry of 

 our own earth and of other worlds once supposed to be beyond 

 the chemist's ken, not only demonstrates the very wide diffu- 

 sion of various chemical elements here on the earth, but shows 

 us that very many of them exist in the sun. If we accept, as 

 most of us are now inclined to do, the nebular hypothesis, and 

 admit that our earth was once, like the sun of to-day, an in- 

 tensely heated vaporous mass ; that it is, in fact, a cooled and 

 condensed portion of that once great nebula of which the sun 

 is also a part, we might expect to find all the elements now 

 discovered in the sun distributed throughout this consolidated 

 globe. We may speculate about the condensation of some of 

 these before others, and their consequent accumulation in the 

 inner parts of the earth ; but the fact that we have all the ele- 

 ments of the solar envelope (together with many more) in the 



