o 



35 



the change. In course of time, the earthy mass falls to the floor 

 by its own weight, aided, perhaps by occasional tremors of the 

 ground, or is detached by the load of stalactites suspended from 

 it below, and thus comes within the levelling and transporting 

 action of the streams flowing through the cave. 



As to the production of the nitrates with which the Petre dirt 

 is more or less impregnated, Prof. R. thought that it could not, 

 in any large degree, be referred to the excretions and other re- 

 mains of animals occasionally found in these caves ; since the 

 quantity of nitrogen required for this purpose would far exceed 

 such a means of supply. Besides this, the nitrates are found in 

 the earthy mass while it is still adhering to the roof or walls, and 

 far removed from the organic matter supposed to be buried in 

 the floor. Nor can we regard the nitrogen as chiefly derived 

 from organic substances in the decomposing rocks. For, in the 

 case of some caves producing Petre dirt, the surrounding lime- 

 stone contains only a trace of such ingredients. We must, there- 

 fore, refer the formation of the nitric acid, and ultimately the 

 nitrates, to mutual chemical reactions between the porous calca- 

 reous earth and the contiguous atmosphere. 



Dr. C. T. Jackson thought that, in connection with this 

 subject, an explanation of the manner of fornnation of stalac- 

 tites might be interesting to those not particularly con- 

 versant with their growth. 



In the grotto at Corneal, near Trieste, where he had observed 

 their formation, the first thing to be noticed was a minute pore 

 or perforation in the rock wall, in which a drop of water, satu- 

 rated with bi-carbonate of lime, had collected. Upon the evapo- 

 ration of the water, the carbonic acid escapes, and the solid 

 material is deposited in the form of a thin ring or hollow crust. 

 Subsequent depositions convert the ring into a tube with very 

 thin walls, and this tube gradually becomes elongated by addi- 

 tional depositions of bi-carbonate of lime. The stalactite then 

 increases externally, commencing at Us upper extremity, and 

 acquires a conical form ; and it may continue to advance until it 

 meets the stalagmite beneath it, which, as is well known, is 

 formed by drippings from the surface of the stalactite. 



