OXYGEN AND ITS SALINE COMBINATIONS 163 



In its chemical properties oxygen is remarkable from the fact that 

 it very easily and, in a chemical sense, vigorously reacts on a number 

 of substances, forming oxygen compounds. However, only a few 

 substances and mixtures of substances (for example, phosphorus, copper 

 with ammonia, decomposing organic matter, aldehyde, pyrogallol with 

 an alkali, <kc.) combine directly with oxygen at the ordinary 

 temperature, whilst many substances easily combine with oxygen at a 

 red heat, and often this combination presents a rapid chemical reaction 

 accompanied by the evolution of a large quantity of heat. Every 

 reaction which takes place rapidly, if it be accompanied by so great an 

 evolution of heat as to produce incandescence, is termed combustion. 

 Thus combustion ensues when many metals are plunged into chlorine, 

 or oxide of sodium or barium into carbonic anhydride, or when a spark 

 falls on gunpowder. A great many substances are combustible in 

 oxygen, and, owing to its presence, in air also. In order to start 

 combustion it is generally necessary 22 that the combustible substance 

 should be brought to a state of incandescence. When once started 

 i.e., when once the incandescent portion of the substance begins to 

 combine with oxygen then combustion will proceed uninterruptedly 

 until either all the combustible substance or all the oxygen is consumed. 

 The continuation of the process does not require the aid of fresh 

 external heat, because sufficient heat 23 is evolved to raise the tempeTa- 

 ture of the remaining parts of the combustible substance to the required 



position of water by weight may be deduced from the densities of hydrogen and oxygen, 

 and the volumetric composition of water. This kind of mutual and opposite correction 

 is a method which strengthens the practical data of the exact sciences, whose 

 conclusions require, above all things, the greatest possible exactitude and variety of 

 corrections. 



It must be observed that the specific heat of oxygen at constant pressure is 0'2175, 

 consequently it is to the specific heat of hydrogen (8'409) as 1 is to 15'6. Hence, the 

 specific heats are inversely proportional to the weights of equal volumes. This signifies 

 that equal volumes of both gases have (nearly) equal specific heats that is, they require 

 an equal quantity of heat for raising their temperature by 1. We shall afterwards con- 

 sider the specific heat of different substances more fully, and we will not, therefore, linger 

 over it at present. 



Oxygen, like the majority of difficulty-liquefiable gases, is but slightly soluble 

 in water and other liquids. At the ordinary temperature, 100 volumes of water dissolve 

 about 3 volumes of oxygen, or more exactly, at 4'1 vols., at 10 8'3, and at 20 3*0 

 (measuring the volumes at the same temperature as the water). From this it is evident 

 that water standing in air must absorb i.e., dissolve oxygen. This oxygen serves for 

 the respiration of fishes. Fishes cannot exist in boiled water, because it does not contain 

 the oxygen necessary for their respiration (see Chap. I.). 



-- ( Vrtain substances (with which we shall afterwards become acquainted), however, 

 inflame of themselves in air ; for example, impure phosphuretted hydrogen, silicon, 

 hydride, zinc ethyl, and pyrophorus (very finely divided iron, &c.). 



-"' If so little heat is evolved that the adjacent parts are not heated to the tempera- 

 ture of combustion, then combustion will cease. 



M 2 



