44-(14) MINERAL PHYSIOLOGY. 



trated by the familiar case of albumen, which, as found 

 in the serum of the blood or the white of an egg, is 

 readily soluble, but by a gentle heat coagulates, and then 

 becomes a white insoluble solid. Among mineral species 

 capable of assuming such a soluble condition are the 

 constituents of some of the hardest and most insoluble 

 crystals found in veinstones. Rock-crystal or quartz, 

 the ruby or corundum, tinstone, rutile and specular iron 

 ore are examples of this ; since the silica, alumina, and 

 the oxyds of tin, titanium and iron, of which they are 

 severally composed, are all readily obtainable in condi- 

 tions in which pure water is capable of dissolving several 

 hundredths of its weight of them. Many other oxydized 

 bodies, more complex in composition than these, are 

 susceptible of assuming a similar condition, and the same 

 is true of even of metallic sulphids, as sulphid of arsenic. 

 It is certain that this capacity of assuming a soluble 

 condition is much more general than has been supposed, 

 and it may here be noticed that although the soluble 

 forms are generally what is called collodial or jelly-like 

 in character, there are many examples of bodies where 

 temporary solubility is apparently independent of a col- 

 lodial condition. 



Again, it is well known that heat in many cases favors 

 solution, as in the case of sugar, alum and most salts. 

 There are, however, not wanting instances in which an 

 increase of temperature does not augment and even di- 

 minishes the solvent power of water. Hence, while most 

 watery solutions saturated when hot deposit crystals of 

 the dissolved body in cooling, there are other solutions, 

 which when saturated in the cold deposit crystals if 

 the temperature is raised. Examples of this are seen in 

 gypsum and in other sulphates. The presence of many 

 alkaline and neutral salts in water in like manner serves 

 to increase its solvent power for certain bodies, while in 

 other cases their presence is not less efficacious in pre- 

 venting solution. Sulphuretted hydrogen and carbonic 

 acid also augment the solvent power of water for many 

 substances. 



In considering the solvent power of waters within the 

 earth's crust, we must farther remember that as we go 

 downward there is a more or less regular increase of tem- 

 perature, equal to about one degree of Fahrenheit for 



