224 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



Let us now compare the waters of rivers, seas, and subter- 

 ranean springs, thus impregnated with various chemical ele- 

 ments, with the blood which circulates through our own bodies. 

 The analysis of the blood shows it to contain albuminoids 

 which go to form muscle, fat for the adipose tissues, phosphate 

 of lime for the bones, fluorides for the enamel of the teeth, 

 sulphur, which enters largely into the composition of the hair 

 and nails, soda which accumulates in the bile, and potash, 

 which abounds in the flesh-fluid. All of these are dissolved in 

 the blood, and the great problem for the chemical physiologist 

 is to determine how the living organism gathers them from 

 this complex fluid, depositing them here and there, and giving 

 to each part its proper material. This selection is generally 

 ascribed to a certain vital force, peculiar to the living body. 

 I shall not here discuss the vexed question of the nature of 

 the force which determines the assimilation from the blood 

 of these various matters for the needs of the animal organism, 

 further than to say that modern investigations tend to show 

 that it is only a subtler kind of chemistry, and that the study 

 of the nature and relation of colloids and crystalloids, and of 

 the phenomena of chemical diffusion, promises to subordinate 

 all these obscure physiological processes to chemical and physi- 

 cal laws. 



Let us now see how far the comparison which we have made 

 between the earth and an animal organism will help us to 

 understand the problem of the distribution of minerals in 

 nature ; how far water, the universal solvent, acting in accord- 

 ance with known chemical and physical laws, will cause the 

 separation of the mixed elements of the earth's crust, and their 

 accumulation in veins and beds in the rocks. The subject is 

 one of great importance to the geologist, who has to consider the 

 genesis of the various rocks and ore-deposits, and the relations, 

 which we are only beginning to understand, between certain 

 metals and particular rocks, and between certain classes of ores 

 and peculiar mineralogical and geological conditions. It is at 

 the same time a vast one, and I can now only give you a few 

 illustrations of the chemistry of the earth's crust, and of the 



