

AIR AND OXYGEN 33 



or even at 357 (its m.-p.) to be of any practical value. Sub- 

 stances which hasten a chemical action without themselves under- 

 going any permanent change are called contact agents, catalytic 

 agents, or catalysts. The process is called contact action or cataly- 

 sis (Greek, decomposition, not a very fortunate choice of words). 

 Such substances are frequently used in chemistry. The addition 

 of a suitable catalyst is one of the conditions (p. 7) for carrying 

 out actions in which a contact agent is necessary. Many sub- 

 stances of this class are secreted by animals and plants and play 

 an important part in digestion, fermentation, and other physio- 

 logical changes. Their presence often enables very complex 

 chemical actions to proceed rapidly at rather low temperatures. 



The oxone, mentioned above, always contains a catalyst, 

 a trace of cuprous oxide, which hastens the action on water. 



Specific Properties of Two Kinds, Physical and Chemical. 



We have learned that every substance has its own set of specific 

 properties. In describing a substance, it is convenient to divide 

 the properties into two classes. The list of substances with 

 which the given substance can enter into chemical combination, 

 for example, we place under specific chemical properties. Rela- 

 tions of the substance to any of the varieties of chemical change 

 belong to this class. 



On the other hand, we do not consider melting or boiling to be 

 chemical changes, so we place the temperatures at which the sub- 

 stance melts (m.-p.) and boils (b.-p.), its color, etc. (for list, see p. 

 6), under specific physical properties. 



Properties of either class may be used for recognizing a substance. 



Specific Physical Properties of Oxygen. Oxygen resembles 

 air in having neither color, taste, nor odor. The density of a sub- 

 stance is, strictly speaking, the weight of 1 cubic centimeter (1 c.c.). 

 In the case of a gas, we frequently prefer to give the weight of 

 1000 c.c. (1 liter), at and 760 mm. (1 atmosphere) barometric 



