308 TESTING BUILDING MATERIALS. 



and it was finally concluded that it wa& due to the previous absorp- 

 tion by the marble of water holding in solution a small portion of 

 organic 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, be exposed 

 to the atmosphere ; evaporation 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 evapora- 

 tion. 



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 water impregnated with impurities. 

 The 23ores of the stones, so long as the blocks remain in the yard, are 

 in the condition of the tube not supplied at its lower end with water, 

 and consequently no current takes place through them, and the amount 

 of evaporation is comparatively small ; but when the same blocks are 

 placed in the wall of the building, the absorbed water from the mortar 

 at the interior surface gives the supply of the liquid necessary to carry 

 the coloring materials to the exterior surface, and deposit it at the 

 outer orifices of the pores. 



The cause of the phenomenon being known, a remedy was readily 

 suggested, which consisted in covering the surface of the stone to be 

 embedded in mortar with a coating of asphaltum. This remedy has 

 apparently proved successful. The discoloration is gradually disap- 

 pearing, and in time will probably be entirely imperceptible. 



This marble, with many other specimens, was submitted to the 

 freezing process fifty times in succession. It generally remained in 

 the freezing mixture for twenty-four hours, but sometimes was I'rozen 

 twice in the same day. The quantity of material lost was .00315 

 parts of an ounce. On this data Captain Meigs has founded an in- 

 teresting calculation which consists in determining the depth to which 

 the exfoliation extended below the surface as the effect of its having 

 been frozen fifty times. He found this to be very nearly the ten thou- 

 sandth part of an inch. Now, if we allow the alternations of freezing 

 and thawing in a year on an average to be fifty times each, which, in 

 this latitude, would be a liberal one, it would require ten thousand 

 years for the surface of the marble to be exfoliated to the depth of one 

 inch. This fact may be interesting to the geologist as well as the 

 builder. 



Quite a number of different varieties of marble were experimented 

 upon. A full statement of the result of each will be given in the 

 reports of the committees. 



At the meeting of the Association at Cleveland, I made a commu- 

 nication on the subject of cohesion. The paper, however, was pre- 

 sented at the last hour ; the facts were not fully stated, and have 

 never been published. I will, therefore, occupy your time in briefly 



