INTRODUCTION 19 



which is derived from the Greek and means "the light-bearer." 

 The emission of light is due to slow oxidation, and although the 

 glow occurs at low temperatures, heat is evolved, and often raises 

 the temperature to a point high enough to start rapid combus- 

 Mon (about 60° C), and actually to set fire to the phosphorus. 

 Phosphorus burns in air with a dazzling white light, evolving 

 dense clouds of phosphorus pentoxide, ^fi^, which readily dis- 

 solve in water, forming phosphoric acid, HgPO^. Phosphorus 

 is a violent poison. It is largely used in the manufacture of 

 lucifer matches, and occasionally as a rat-poison. From an 

 agricultural standpoint its chief importance lies in the use of 

 its compounds, the phosphates, as manures, and its occurrence 

 in association with fats and albuminoids in feeding stuffs and 

 in the bodies of animals. 



Calcium is very abundant in nature, always 'occurring 

 in a combined state. Calcium carbonate, CaC03, is found in 

 enormous quantities in chalk, limestone and marble ; the sul- 

 phate is very abundant as gypsum or selenite, CaSO^ .2Kfl; 

 and the silicate is found as a constituent of many minerals. 

 The element itself is an easily oxidisable metal, difficult to 

 prepare, and of little importance. Its oxide, CaO, is the 

 important substance quicklime. Calcium is an essential con- 

 stituent of plant food, but in soil its compounds fulfil other 

 more important functions, which will be described hereafter. 



Potassium occurs in many minerals. Most silicates con- 

 tain it in smaller or larger amount, and in some, e.g., oriho- 

 clase, AlgOg.KjO.GSiOg, and mica, K20.3A1^03.4Si02, potassium 

 is an essential ingredient. It also occurs in sea-water, from 

 which sea-weeds accumulate large quantities of potassium 

 compounds. The huge saline deposits at Stassfurt furnish a 

 large proportion of the potassium required in the arts and for 

 manurial purposes. The character of these deposits will be 

 described later. 



The element can be prepared by the action of carbon at a 



