AMOUNT OF WATER-GAS DETERMINED BY HEAT. 



17 



withdraw as much gas as they can hold. From raius which thus take place and fall on 

 elevated regions, springs and rivers arise, containing, therefore, those gaseous materials 

 which have been obtained from the atmosphere. 



44. This water-gas, as it might be termed, may be expelled entirely from water by 

 boiling, and by resorting to that process a knowledge of its constitution has been obtained 

 (Ap., 93). The power by which liquids hold gaseous substances in solution is diminished 

 by heat, and as soon as a liquid boils all extraneous gas is carried away, because, under 

 those circumstances, the vapour which is generated by the heat, passing in bubbles 

 through the mass of the liquid, exposes itself to the contained extraneous gas, which 

 diffuses rapidly into it, and escapes away when the bubbles burst on the top. The law 

 of equilibrium of diffusing gases teaches us that this process can only end in the total 

 removal of the contained air (Ap., 47, 48). Temperature thus controlling the quantity 

 of gas dissolved in water, we can easily see that in different countries the relative 

 amount of water-gas will differ, and that its mean quantity will be connected with the 

 mean annual temperature, and in those seasons during which vegetable action is ad- 

 vancing most rapidl3 r , that is, in spring and summer, its amount will be determined by 

 the mean temperature of the spring and summer months. In tropical climates there 

 should be less of this gas than in temperate, but in the use which is to be made of it 

 for physiological purposes a compensating agency appears ; the poorer water of equa- 

 torial countries is acted on by a more brilliant ray. The light and heat of the sun 

 here stand iu the attitude of antagonizing forces. The calorific beams, by reason of their 

 obliquity iu the more polar countries, allow an increased quantity of gas to be dissolved 

 iu water ; the luminous beams which come with the same obliquity have therefore more 

 to operate on. 



45. If saline matter be present, it acts as a disturbing agent ; for a liquid which is 

 impregnated with salt has its solvent powers greatly diminished. Bulk for bulk, there- 

 fore, sea water must contain a less volume of gas than fresh water. 



4G. The gas thus included in the interstices of water discharges many important 

 functions. It is connected with the physiological operations both of the animal and 

 vegetable world. Being twice as rich in oxygen as atmospheric air (Ap., 93), it is des- 

 tined to carry on the respiratory processes of those classes of animals whose form re- 

 quires that their breathing apparatus should be of the most compendious figure, and 

 which have to live in the water. Thus, in those fishes which breathe by branchiae, the 

 processes of combustion which are to go on in their systems depend on a supply of the 

 concentrated oxygen of water-gas. The fish swallows a mouthful of water, and then, 

 by muscular contraction, drives it out past the gills, among the threadlike fibrilla? in 

 which venous blood is passing. Inter-transfusion in an instant takes place, oxygen 

 from the water-gas flashes into the venous blood and arterializes it, carbonic acid si- 

 multaneously comes out, and, as the fish moves forward, is carried away in the current 

 of water. 



47. A mass of water thus containing carbonic acid, oxygen, and nitrogen, with some 

 germ, or sporule, or objective point, on which the light can act, is exposed to the sun. 

 As has been said, a bubble of gas soon makes its appearance, and growth, with a devel- 



C 



