18 TWO DECOMPOSITIONS EFFECTED. 



opment of the green colour, takes place. If we examine the changes which are now 

 occurring in the water, we find that the carbonic acid is disappearing, and oxygen and 

 nitrogen are evolving. The growing mass increases in volume and weight. After a 

 time, if proper measures are taken to cut off any farther supply of carbonic acid, the pro- 

 cess conies to an end. But as conducted naturally, there is a free exposure to the at- 

 mosphere, from which the acid diffuses, and hence, as fast as its removal by decompo- 

 sition takes place, new quantities are restored again. 



48. The direct action which is accomplished under the influence of the light is the total 

 decomposition of the carbonic acid gas. Its oxygen is evolved, and its carbon goes to 

 form the green matter. At first sight, the supposition of the generality of chemical 

 writers seems to be probable, that the carbon thus evolved, while it is yet in a nascent 

 state, unites directly with the elements of water, and produces one of the starch family 

 of bodies. But the constitution of the green matter is far from being so simple. 

 Chemical examination, and considerations connected with the nutrition of the insects 

 which feed on it, serve to show that it contains more hydrogen than is necessary to 

 convert its oxygen into water. There is also nitrogen in it. 



49. In the air-bubbles which form and finally escape, we find not pure oxygen, as is 

 generally supposed, but a mixture of oxygen and nitrogen gases. The source of the 

 former is unquestionably to be referred to the decomposed carbonic acid. From the 

 circumstance that in the growing mass there is an excess of hydrogen, and in the esca- 

 ping air a large quantity of azote, it would seem that a compound having the same ele- 

 ments as ammonia has been acted upon. Nevertheless, the quantity of ammonia which 

 is commonly dissolved in spring or river water is far too small to furnish the supply 

 which would be necessary. It seems, therefore, not improbable that the nitrogen nat- 

 urally dissolved along with the carbonic acid and oxygen in the water is indirectly 

 connected with the decomposition. 



50. The presence of carbon, and an excess of hydrogen in the green matter, proves 

 that the sunshine has effected two different decompositions, the decomposition of car- 

 bonic acid and that of water. 



51. At this stage of the description, let us pause and review the facts which have 

 presented themselves to us. We have arrived at the conclusion that all the solid ma- 

 terial under consideration is produced from gaseous matter contained in water in a state 

 of solution. This gaseous matter comes directly from the, atmosphere ; it contains the same 

 constituents as the atmosphere, but differs from it in having them in a different proportion. 

 We see, therefore, that there are, as it were, two atmospheres enveloping the earth's sur- 

 face; one of these is the air which we breathe, the other that which exists dissolved in the 

 sea, and in those streams and waters that are found on the surface of the ground. Between 

 these a remarkable relation exists : in the common atmosphere the proportion of oxygen 

 to nitrogen is nearly as one to four, in the water-atmosphere it is nearly as one to two. 



52. Between these two atmospheres, a constant connexion is established, through 

 the chemical relations of water, which receives its gaseous contents from several differ- 

 ent sources. Part of its carbonic acid it obtains by direct absorption from the superin- 

 cumbent air, and part is given to it by the respiration of fishes and other aquatic ani- 



