352 WRITINGS OF JOSEPH HENRY. [1855 



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 

 material 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 asphal- 

 tum. This expedient has apparently proved successful. The 

 discoloration is gradually disappearing 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 frozen twice in the same day. The quantity 

 of material lost was "00315 part of an ounce. On these data 

 Captain M. C. Meigs has founded an interesting calculation, 

 which consists in determining the depth to which the exfolia- 

 tion extended below the surface as the effect of its having been 

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

 thousandth part of an inch. Now, if we allow the alterations 

 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 

 ma}'" be interesting to the geologist as well as to the builder. 



Quite a number of different varieties of marble were ex- 

 perimented upon. A full statement of the result of each will 

 be given in the reports of the committees. 



On molecular Cohesion. 



At the meeting of the Association at Cleveland I made a 

 communication on the subject of Cohesion* The paper how- 



* [Proceedings of the American Association for the Advancement of 

 Science, July, 1853; vol. vii, p. 270. Only the title published.] 



