OXVHKN AND JTS SALINE COMBINATIONS 158 



proceed around us, are also very often dependent on the action of the 

 oxygen of the air, and also reduce it from a free to a combined state. 

 The majority of the compounds of oxygen are, like water, very stable, 

 and do not give up their oxygen under the ordinary conditions of nature. 

 As these processes are taking place everywhere, therefore the amount 

 of free oxygen in the atmosphere should decrease, and this decrease 

 should proceed somewhat rapidly. This is, in fact, observed where 

 combustion or respiration proceeds in a closed space. Animals suffocate in 

 a closed space because in consuming the oxygen the air remains unfit for 

 respiration. In. the same manner combustion, in time, ceases in a closed 

 space, which may be proved by a very simple experiment. An ignited 

 .substance for instance a piece of burning sulphur has only to be placed 

 in a glass flask, which is then closed with a stout cork to prevent the 

 access of the external air ; combustion will proceed for a certain time, 

 so long as the flask contains any free oxygen, but it will cease, although 

 there still remain unburnt sulphur, when all the oxygen of the enclosed 

 air has combined with the sulphur. From what has been said, it is 

 evident that regularity of combustion or respiration requires a con- 

 stant renewal of air that is, that the burning substance or respiring 

 animal should have access to a fresh supply of oxygen. This is attained 

 in human habitations by having many windows, outlets, and ventilators, 

 and by the current of air produced by tires and stoves. As regards the 

 air over the entire earth's surface, its amount of oxygen hardly decreases, 

 because in nature there is a process going on which renews the supply 

 of free oxygen. Plants, or rather their leaves, during daytime 3 that is, 

 under the influence of light evolve free oxygen. Thus the loss of 

 oxygen which occurs in consequence of the respiration of animals and of 

 combustion is made good by plants. If a leaf be placed in a bell jar con- 

 taining water, and carbonic anhydride (because this gas is absorbed and 

 oxygen evolved from it by plants) be passed into the bell, and the whole 

 -apparatus be placed in sunlight, then oxygen will accumulate in the 

 bell jar. This experiment was first made by Priestley at the end of the 

 last century. Thus the life of plants on the earth not only serves for 

 the formation of food for animals, but also for keeping up a constant 

 percentage of oxygen in the atmosphere. In the long period of the life of 

 the earth that equilibrium has been attained between the processes ab- 



" A t night, without the action of light, without the absorption of that energy which 

 is required for the decomposition of carbonic anhydride into free oxygen and carbon, 

 which is retained by the plants, they breathe like animals, absorbing oxygen and evolving 

 carbonic anhydride. This process also goes on side by side with the reverse process in 

 daytime, but then it is far feebler than that which gives oxygen. This observation is a 

 necessary consequence of an aggregate of data referring to the physiological processes of 

 plants. 



