144 NON-METALS AND THEIR COMBINATIONS. 



67,883 calories of heat are liberated, or enough to raise nearly 68 kilogrammes 

 of water one degree in temperature. The thermal equation, 



HgO = Hg -t O 30,370.5 cal., 



means that when 214.38 grammes of oxide of mercury (corresponding to HgO) 

 are decomposed by heat into mercury and oxygen, 30,370.5 calories of heat 

 are absorbed and converted into chemical energy which is associated with the 

 elements mercury and oxygen. The plus sign is used when heat is liberated in 

 the formation of a compound, and the latter is termed exothermic; while the 

 minus sign indicates absorption of heat, and the compound is termed endother- 

 mic. Exothermic compounds are relatively stable, while endothermic ones are 

 unstable and often explosive. They decompose easily with liberation of heat. 



Ozone is endothermic, as heat is absorbed during its formation from oxygen. 

 When it decomposes heat is liberated. The thermal equation, 2O 3 3O 2 + 

 64,314 cal., states that when 95.28 grammes of ozone (corresponding to 2O 3 ) 

 decomposes into ordinary oxygen, 64,314 calories of heat are liberated. The 

 greater chemical energy of ozone over that of oxygen accounts for its greater 

 chemical activity as compared with oxygen. 



Thermo-chemical measurements are of great importance in several practical 

 directions; for example, for determining the fuel values of samples of coal, 

 coke, wood, fuel values of articles of food in the field of physiology, etc. 



11. HYDROGEN. WATER. HYDROGEN DIOXIDE. 

 H = 1. H 2 O = 17.88. H 2 O 2 = 33.76. 



History. Hydrogen was obtained by Paracelsus in the 16th cen- 

 tury ; its elementary nature was recognized by Cavendish, in 1766. 

 The name is derived from Mvp (hudor), water, and yewfo (gennao), to 

 generate, in allusion to the formation of water by the combustion of 

 hydrogen. 



Occurrence in nature. Hydrogen is found chiefly as a component 

 element of water ; it enters into the composition of most animal and 

 vegetable substances, and is a constituent of all acids. Small quanti- 

 ties of free hydrogen are found in the gases produced by the decom- 

 position of organic matters (as, for instance, in the intestinal gases), 

 and also in the natural gas escaping from the interior of the earth. 



QUESTIONS. By whom and at what time was oxyge'n discovered? How is 

 oxygen found in nature? Mention three processes by which oxygen may be 

 obtained. How much oxygen may be obtained from 490 grammes of potas- 

 sium chlorate? State the physical and chemical properties of oxygen. Ex- 

 plain the terms combustion, slow combustion, combustible substance, and sup- 

 porter of combustion. Mention some oxidizing agents. What is ozone, and 

 how does it differ from common oxygen? Under what circumstances is ozone 

 formed? What is thermo-chemistry ? What is a thermal reaction? 



