THE NON-LIVING WORLD 141 



heat will render it liquid, and a little more will make 

 it aeriform. Then, if a cold surface be brought near, 

 it will again become solid, being deposited on that 

 surface in the form of minute grains, known as " flour 

 of sulphur." But besides its liquid and gaseous states, 

 sulphur may exist in more than one solid condition, and 

 may be made to pass alternately backwards and for- 

 wards, from one solid condition to the other, by means 

 of slight changes of temperature. One of these is known 

 as crystalline sulphur, while the other is non-crystalline.* 

 Substances which'crystaUise in two different forms, with- 

 out change of nature, are called dimorphic; and those 

 which, like carbon, can exist in more than two, are termed 

 polymorphic. The same body may possess distinct pro- 

 perties, and any one such state contrasted with another, 

 is said to be an " allotropic " state. Thus, we say crystal- 

 line sulphur can exist in an " allotropic " state which is not 

 crystalline. Sulphur forms some very notable substances 

 with the aid of other elements. Thus, sulphuric acid, or 

 " oil of vitriol," is an oleaginous liquid, formed by three 

 equivalents of oxygen, one of sulphur, and one of 

 water. Sulphuric acid forms, with various other sub- 

 stances, certain matters termed sulphates, as sulphate 

 of copper, sulphate of magnesia, sulphate of lime, &c. 

 It takes away the citric acid from citrate of lime, 

 seizing on the latter and forming sulphate of lime, the 

 citric acid being thus left free. The termination "ic" 

 signifies that the body so distinguished has more oxygen 

 than one distinguished by the termination " ous " as 

 sulphuric acid has more than sulphurous acid has. Sul- 

 phurous acid is formed by two equivalents of oxygen and 

 one of sulphur. It produces that powerful, suffocating 



* As to what a crystal is, see post, p. 143. 



