16 CHEMISTRY. 



The subject of the crystallization of minerals belongs to min- 

 eralogy, and will be fully treated in Part Third. I shall 

 barely allude to it here. You see the tendency spoken of 

 in almost every mineral, and it never fails in its operation 

 except from opposing circumstances. You can often see it 

 in the rudest stone, especially if you call to your aid the 

 microscope. The angles and edges and faces of the half- 

 formed crystals can be seen huddled together. In the rocks 

 and mountains we see this crystalline tendency roughly ex- 

 hibited in lamina and pillars. The most common exhibi- 

 tion of it is furnished us in water as it solidifies into snow 

 and frost and ice. 



9. Relations of Heat to the Forms of Substances. Most 

 substances, whether elementary or compound, like mercury 

 and bromine, exist in different forms at different temper- 

 atures. We are accustomed to speak of them in the form 

 in which they usually appear to us, with the idea that this 

 is their natural condition. And yet this condition depends 

 wholly upon circumstances. Alter the temperature vari- 

 ously, and you may have them solid, liquid, or dissipated in 

 the form of vapor. Thus we speak of iron as a solid, and 

 mercury as a liquid ; but you can heat iron so as to make 

 it a liquid, and you can cool mercury so as to make it a solid. 

 Indeed, in some parts of the earth, the extreme arctic re- 

 gions, the natural condition of mercury is that of a solid. 

 Then, too, you can by heat turn mercury, heavy as it is, into 

 a vaporous or gaseous condition. Water exists in the three 

 different forms, solid, liquid, and gaseous, according to the 

 degree of heat. Some substances can exist in only one 

 form, so far as we know. This is the case with some of the 

 gases. Some substances can exist in but two forms. Thus 

 alcohol can be only in the liquid and gaseous forms, the 

 severest cold which man has ever produced not having 

 been able to make it solid. 



