INFLUENCE OP BACTERIA. 303 



I know of no determinations of the carbonic acids in the soils of Brazil 

 such as were made by Boussingault and Lewy in France.* 



Supposed work of bacteria. — In the original preparation of this paper 

 the work of bacteria in rock decomposition was not specifically referred 

 to because it was supposed to be included under the general head of 

 organic acids derived from the decay of plants. Since the paper was 

 presented I have found that there is an impression abroad that bacteria 

 attack rocks directly, and that they must be the more important in 

 tropical countries on account of their greater activity in warm, moist 

 climates. 



It is not possible to discuss this subject at length here, but a brief 

 statement of the principal facts will not be out of place. It was formerly 

 supposed that only the chlorophyl-bearing plants could subsist upon 

 mineral substances, and that organic matter was indispensable to the 

 existence of bacteria. Warming definitely states that " organic carbon 

 compounds are indispensable for all bacteria except, as it appears, for 

 the nitrifying organisms." f In 1886 Frankland began experiments 

 which demonstrated the fact that certain forms of the nitrifying bacteria 

 may live without organic food. J Similar results were obtained by War- 

 ing, § Winogradsky || and Berthelot. ^ It cannot be Considered as dem- 

 onstrated from these researches, however, that bacteria are capable of 

 taking their sustenance directly from the rock-making minerals. The 

 solutions used in the experiments in every case contained substances 

 from which the bacteria could derive the nitrogen (and carbon) essential 

 to their existence, such as humic acid, ammonium chloride, or some 

 other form or combination of nitros-en. No such substances occur in the 

 rocks except as they are carried in from the surface, and when so intro- 

 duced the bacteria which go with them simply take part in the attack 

 made upon the rock constituents by organic acids. It is easy to under- 

 stand, then, why Muntz found bacteria at depths in " des roches dites 

 pourries," or disintegrated rock;** they were there chiefly as the result 

 of decay, not as the cause of it. In brief, I am unable to find any evi- 

 dence whatever that bacteria attack rocks directly. The processes of nitri- 

 fication and decomposition performed by bacteria are soil and surface 

 phenomena in the main, and while these phenomena may penetrate to 



* Memoire sur la composition de I'air confine dans la terre vej>;etale. Boussingault et Lewy. 

 Com^t. Rend., 35, 1852, pp. 765-771. 



t A Handbook of Systematic Botany. E. Warming. Translated by M. C. Potter. London, 1895, 

 pp. 31, 32. 



X Phil. Trans. Roy. Soc, 1890, B., p. 107; Nature, vol. xlvi, 1892, pp. 136-138. 



§ Annales de I'lnscitut Pasteur, 1890, p. 268. 



II Exper. Station, Bui. 8, U. S. Dept. Agr., 1892, p. 42 et seq. 



^Compt. Rend., 1893, cxvi, pp. 812-819. 



**Compt. Rend , ]890, ex, p. 1370. 



XXXIII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



