Study of the New York Obelisk as a Decayed Boulder. 159 



which attend chemical decay, tend to develop mainly the micro- 

 scopic clefts in the interior of mineral-grains. 



The rock-pores connect in chains of easily communicating voids, 

 forming an intricate network which freely imbibes water, by capil- 

 lary attraction, from any moistened surface, until completely filled. 

 The communication between the mineral- clefts is interrupted and 

 difficult, and their occupation by liquid is slow, on account partly 

 of their minuteness and partly of their content of air, probably as 

 a condensed film. The distinction of the two classes of voids, of 

 their origin, and of conclusions from their proportion, seems to me 

 important. 



The apparatus and process employed for the purpose need to be 

 first described. After some modifications, they were applied by 

 me some years ago to a long series of trials on building-stones of 

 this country, and were found to yield uniform and satisfactory re- 

 sults. 



The main apparatus consists of a low bell-jar, 12 inches in diame- 

 ter and 6 inches in height, with glass knob for convenient handling; 

 this stands in about half an inch of distilled water in a large shallow 

 tray. Within the bell-jar and half immersed in the water, is a 

 round, soft clay tile, with even and smooth upper surface, 9 inches 

 in diameter and about 1 inch in thickness. Before use, this tile 

 must be repeatedly boiled in distilled water to remove all soluble 

 matter from its interstices. 



On the top of the tile several pads of sheet-rubber, 3 to 4 inches 

 across, are laid. In the centre of each pad a square opening, 1 

 inch on a side, is occupied by a pad of thick soft blotting-paper, 

 which, of course, remains constantly saturated with water drawn 

 up from the tile. Each of the rubber pads is also kept covered with 

 a small low glass cover or inverted dish, to prevent the fall of con- 

 densed water from the vault of the bell-jar. The water lost by 

 evaporation outside the bell-jar is constantly replaced, so as to keep 

 a constant level. Without a suitable precaution, the raising of the 

 bell-jar from the water would be accompanied by a sudden inrush 

 of water and flooding of the tile. This is prevented, either by a 

 short bent piece of glass tubing, which passes from outside down and 

 around the edge of the bell-glass and so up into its interior, so as 

 to provide constant communication between the air outside and in ; 

 or more conveniently by a half-inch hole bored through the vault of 



