THE GROWTH OF STALAGMITES AND STALACTITES 109 



channels in the stalactite wall. After drip-evaporation equilibrium 

 is attained, the stalactite continues to grow symmetrically, (with 

 uniform diameter) in the vertical direction, unless conditions change. 



The removal of the carbonic acid from the pendant drop of lime 

 solution causes the precipitation of the basic carbonate of lime as a 

 thin, translucent film on the surface of the drop. If evaporation is 

 good, this film is repeatedly broken and the pieces go spinning up 

 to the rim of the tube — the drop is "alive." This spinning motion 

 is due to surface tension and also to the fact that the down-flowing 

 current is at the center of the drop and the up-flowing current at 

 the periphery. Figure 3 shows a drop that is "alive" — good 

 evaporation. Figure 5 shows a drop that is " dead " — ^poor evapora- 

 tion. 



Preceding the breaking away of the drop, there is a periodic, 

 vertical oscillation which increases in frequency and amplitude 

 until the actual break occurs. 



EXAMPLES OF GROWING STALACTITES 



Stalactites are growing under known evaporation conditions 

 at the Experimental Mine of the United States Bureau of Mines, 

 near Bruceton, Pennsylvania, on the concreted roof. Experimental 

 coal-dust explosions are conducted in the mine during fall and 

 winter, when the mine is dry, and high air is used to purge the mine 

 of smoke and dust. The parts of the stalactites which grow during 

 this period are blackened. This allows the rate of growth and the 

 age of the stalactites to be closely determined. Also, certain of 

 the stalactites are growing in the aircourse which is subject to 

 violent explosions when the coal-dust work is under way. They 

 are therefore no older than January 8, 192 1, when the last explosion 

 experiment was made. Unsymmetrical stalactite No. 2 grew in a 

 gallery which was not completed until September 9, 192 1. All 

 these are rapid-growing stalactites of shell-like tube form, 0.4 to 

 0.5 cm. in outside diameter for the small part and about i.o to 

 i.2 cm. for the large part. The walls are about 0.02 cm. thick and 

 are studded on the inside with small excrescences about 0.05 to 

 0.08 cm. thick. The rates of growth of the stalactites are given in 

 Table I. 



