OXYGEN. 3 



bodies of plants and animals, i.e., into nearly all forms of 

 organic matter. Its atomic weight is the smallest of all the 

 elements and was formerly taken as unity, now it is more 

 usual to take oxygen^lG as the basis of atomic weights, that 

 of hydrogen on this scale being about 1-008. Consequently, 

 though the proportion by weight of hydrogen in the substances 

 comprising the crust of the earth is small, yet the number of 

 atoms of hydrogen actually existent and taking part in the 

 changes going on must be very large compared with those of 

 other elements apparently (and by weight) much more abun- 

 dant. Take water for example here the hydrogen by weight 

 constitutes only ^ of the total and the oxygen -|, yet there 

 really are twice as many atoms of hydrogen as of oxygen, as 

 indicated by the formula H 2 0. In reality the relative amounts 

 of elements present in any system so far as their chemical 

 activity is concerned ought to be measured by the respective 

 numbers of atoms present, not by their respective weights. 

 Eegarded in this way, hydrogen is of relatively far greater 

 importance and abundance than is usually estimated (v. p. 19). 



Oxygen is perhaps the most important element known. 

 It is by far the most abundant, and takes part in a greater 

 number of the chemical changes occurring in nature than any 

 other element. It is, indeed, chiefly remarkable for its activity 

 and its power of uniting with almost all other elements. 



Its method of preparation and chief properties are well 

 known to all students of chemistry, being appropriately chosen 

 for consideration early in their course of study. 



Its union with other bodies is usually attended with the 

 evolution of much heat and often light. Present in the free 

 state in air, it plays an important part in the chemical actions 

 attendant upon the processes of respiration, combustion, decay, 

 and almost all the forms of " weathering" which occur around us. 



Oxidation, i.e., union with oxygen, is a process of vital im- 

 portance. The life of animals, especially, may be said to 

 almost consist of oxidation. So, too, the changes occurring in 

 the soil, the "fermenting" of hay, ensilage, &c., the putrefaction 

 and decay of animal matter, and many other processes are 

 largely dependent upon combination with oxygen. Union with 



B 2 



