1855] WRITINGS OF JOSEPH HENRY. 351 



and occasionally also small particles of bisulphuret of iron. 

 Its specific gravity is 2"8620; its weight 178"87 lbs, per cubic 

 foot; it absorbs "103 parts of an ounce per cubic inch, and its 

 porosity is great in proportion to its power of resistance to 

 pressure. It sustains 23,917 lbs. to the square inch. It not 

 only absorbs water by capillary attraction, but, in common 

 with other marbles, suffers the diffusion of gases to take 

 place through its substance. Dr. Torrey found that hydro- 

 gen and other gases, separated from each other by slices of 

 the mineral, diffuse themselves with considerable rapidity 

 through the partition. 



This marble, soon after the workmen commenced placing 

 it in the walls, exhibited a discoloration of a brownish hue, 

 no trace of which appeared so long as the blocks remained 

 exposed to the air in the stone-cutter's yard. Various sug- 

 gestions and experiments were made in regard to the 

 cause of this remarkable phenomenon, and it was finally 

 concluded that it was due to the previous absorption by the 

 marble — of water holding in solution a small portion of or- 

 ganic matter, together with the absorption of another portion 

 of water from the mortar. 



To illustrate the process, let us suppose a fine capillary tube, 

 the lower end of it immersed in water, and of which the 

 internal diameter is sufficiently small to allow the liquid 

 to rise to the top, and be exposed to the atmosphere, evapo- 

 ration will take place at the upper surface of the column, a 

 new portion of water will be drawn up to supply the loss; 

 and if this process be continued any material which may be 

 dissolved in the water, or mechanically mixed with it, will 

 be found deposited at the upper orifice of the tube, or at the 

 point of evaporation. 



If however the lower portion of the tube be not furnished 

 with a supply of water, the evaporation at the top will not 

 take place, and the deposition of foreign matter will not be 

 exhibited, even though the tube itself may be filled with 

 w^ater impregnated with impurities. The pores of the stones, 

 so long as the blocks remain in the yard, are in the condi- 

 tion of the tube not supplied at its lower end with water, and 



