62 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



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early forms, it may be, of several different species of Confervse. That 

 these cells all originate from germs, and not merely from a combina- 

 tion of inorganic elements, appears not only from general considera- 

 tions, but also from the fact that, if measures be taken to free the water 

 entirely from any possible infusion of organic matter, and to admit into 

 contact with it such air alone as has undergone a similar purification, 

 no green flocks make their appearance, under the prolonged influence 

 of the strongest sunlight. We find, then, that the presence of a germ 

 is one of the conditions indispensable to the chemical transformation in 

 question. It may be asked how it can be certainly ascertained that 

 light, and not heat, is the essential condition of this process ; seeing 

 that the two agents are combined in the solar beam. To this it may 

 be replied, that a certain moderate amount of heat is undoubtedly ne- 

 cessary ; but that no degree of heat without light will be effectual in 

 producing the change, as is easily proved by exposing the water to 

 warmth in a dark place. Moreover, when a certain measure of light is 

 afforded, variations in the amount of heat make very little difference ; 

 but we shall presently see that under the same degree of heat, the 

 amount of the change is directly proportional to the intensity of the 

 light. Although, therefore, heat furnishes an essential condition, it 

 cannot be questioned that light is the chief agent in the process, by 

 which the germ brings into union the elements to be employed in the 

 development of its own fabric. 



81. The next question is, What are these elements, and whence are 

 they obtained ? All water that is long exposed to the atmosphere ab- 

 sorbs from it a certain amount of its constituent gases ; but these do 

 not enter it in the proportions in which they are contained in the 

 atmosphere itself ; their relative quantities, in a given measure of water, 

 being proportional to the facility with which they are respectively ab- 

 sorbed by the liquid. Thus, carbonic acid is most readily absorbable ; 

 oxygen next, and nitrogen least so. From the experiments of Prof. 

 Draper, it would appear that, notwithstanding the very small propor- 

 tion of carbonic acid contained in the atmosphere (usually not more 

 than l-2000th part), it forms as much as 29 per cent, of the whole 

 amount of air expelled from water by boiling. Of the residue, one-third 

 consists of oxygen, and the remaining two-thirds of nitrogen ; so that 

 the proportion of the oxygen to the nitrogen is as one to two, instead 

 of being one to four, as in atmospheric air. The absolute quantity of 

 this water-gas, contained in any measure of water, is subject to varia- 

 tion with the temperature ; the quantity being diminished as the tem- 

 perature rises. Now when water thus impregnated with carbonic acid, 

 oxygen, and nitrogen, and containing the germs of aquatic plants, is 

 exposed to the sun's light, a development of vegetable structure takes 

 place, indicated by the green flocculent appearance, as already men- 

 tioned. If the changes, which are now occurring in the water, be ex- 

 amined, we find that the carbonic acid is diminishing in amount ; and 

 that oxygen is being evolved. The growing mass increases in volume 

 and weight ; and after a time exhausts the whole carbonic acid origi- 

 nally contained in the water. If it be then prevented from receiving 

 an additional supply, the process stops ; but as conducted naturally, 



