igoS.] THE PHYSICS OF THE EARTH. 1G5 



under very restricted conditions, so that it is not a general property 

 of matter as zvas once supposed. 



In the early days of physical science the demonstration of the 

 porosity of such dense bodies as gold, silver and lead was considered 

 a great achievement. In 1661 some academicians at Florence, re- 

 peating an earlier experiment of Bacon with a spherical shell of 

 lead, filled a hollow sphere of solid gold with water, and, after 

 sealing it hermetically, flattened the figure of the spherical shell in a 

 hydrostatic press so as to diminish the volume. Under this deforma- 

 tion of the sphere the water was forced through the walls of solid 

 gold and formed in drops on the outside. Corresponding experi- 

 ments were made with spheres of silver, lead, and other metals, 

 with analogous results. ^Modern engineering presents innumerable 

 illustrations of the porosity and leaky character of structures made of 

 the hardest bodies. Under great pressure all pipes and pistons leak, 

 and put a limit to the applications of hydrostatic pressure. 



In 1883 Amagat forced mercury through plates of solid steel 

 three inches thick, under a pressure of about 4,000 atmospheres. 

 This is the highest pressure hitherto applied in physical experiments, 

 and yet all rocks are subjected to such pressure at a depth of only 

 ten miles below the earth's surface. In the measurement of ocean 

 depths it has been found that empty hollow glass balls with walls 

 half an inch thick sent down with the deep sea apparatus come 

 up more and more completely filled with water, according to the 

 depth of the sea and the duration of the experiment. As glass is 

 the most impervious of solid bodies, this leakage, which it shows 

 under the external application of fluid pressure from the deep sea, 

 is a good illustration of what happens to the bed of the ocean, which 

 is constantly subjected to this pressure. No rock is anything like 

 so impervious as glass, and consequently a general leakage of the 

 ocean bottom inevitably takes place. The water which first enters 

 the bed of the sea will keep on descending till it comes into contact 

 with rock at high temperature, which produces and readily absorbs 

 steam. When the rock becomes saturated with steam it swells and 

 requires more space, and this finally brings on an earthquake. 

 Hence also the preponderance of great earthquakes under the sea 

 and the almost total absence of these disturbances far inland. 



