The Romance of Chemistry 735 



change from liquid or gaseous condition into the solid form ; they 

 may be produced, for instance, by the evaporation of saturated 

 solutions. It has been stated as a statistical truth that "everything 

 strives towards symmetry in so far as the environment will allow." 



Diamonds 



A large number of definite chemical bodies are found in 

 nature in crystalline conditions, such as the diamond. The dia- 

 mond is the purest form of crystallised carbon. In the diamond 

 "every carbon atom is symmetrically surrounded by four other 

 carbon atoms, arranged at the corners of a tetrahedron in such 

 manner that the whole crystal is one continuous molecule, thus ex- 

 plaining, as it is thought, its great density and hardness." 



A crystal arouses our interest by reason of the regularity of 

 its form, the perfection of its surfaces and angles, its transparency 

 and brilliancy. A perfect crystal, like a diamond, is quite trans- 

 parent and colourless, although it possesses that marvellous "fire" 

 which we see oscillating with every movement, but most precious 

 stones are tinged with grey, yellow, or brown. It is due to the 

 high qualities of refraction and dispersion of light that diamonds 

 give off the beautiful flashes of blue, gold, and red "fire"; other 

 colours such as ruby red and emerald green are more rare. The 

 colour of many precious stones is really produced by minute pro- 

 portions of impurity. The continued study of the formation of 

 crystals, it is thought, may throw further light upon electrons and 

 their connection with the structure of the atom. 



Colloids 



Another set of facts of first rate importance relates to what 

 is called the colloidal state of matter. In 1861 the Scottish chem- 

 ist Thomas Graham noticed that some substances pass readily 

 through a parchment paper, while others diffuse very slowly 

 through. He called the latter "colloids" (from the Greek kolle 

 =glue) and the former "crystalloids." White of egg and gum 

 may illustrate colloids ; sugar and salts crystalloids. But as many 



