p Human Physiology. 



Water has the power of separating the atoms of the hardest 

 substances lime, silica, salts of various kinds. Water drip- 

 ping from the roofs of caverns covers them with beautiful 

 icicle-like stalactites. But why the forces that give hardness 

 to crystals of salts should cease to act in the presence of water; 

 why atoms of steel should leave each other to make a miser- 

 able rust with oxygen, we know no more than we do why 

 sulphur, saltpetre, and charcoal, which burn quietly apart, 

 should so tremendously explode when brought together, or 

 why a pocket handkerchief, washed in acidulated water and 

 dried, needs but a spark of fire to blow up a house. 



Atoms arrange themselves in certain positions in relation to 

 other atoms in what is called crystallisation, forming in lines 

 like soldiers, and with these lines making cubes of many 

 beautiful forms. In the same matter the forms may vary but 

 the angles are always alike, as may be seen in the snow 

 crystals : 



Fig. I SNOW CRYSTALS. 



The forces that hold atoms in their places in rock crystal 

 or diamond are marvellous; but the forces exerted in the 

 formation of crystals are perhaps still more so. The atoms of 



