1855] WRITINGS OF JOSEPH HENRY. 345 



of a building are to be exposed for centuries, the conclusions 

 desired are to be drawn from results produced in the course 

 of a few weeks. Besides this, in the present state of science 

 we do not know all the actions to which the materials are 

 subjected in nature, nor can we fully estimate the amount 

 of those which are known. 



The solvent power of water, which attacks even glass 

 must in time produce an appreciable effect on the most solid 

 material, particularly where it contains, as the water of the 

 "atmosphere always does, carbonic acid in solution. The 

 attrition of siliceous dusts, when blown against a building, or 

 washed down its sides by rain, is evidently operative in wear- 

 ing away the surface, though the evanescent portion removed 

 at each time may not be indicated by the nicest balance. 

 An examination of the basin which formerly received the 

 water from the fountain at the western entrance of the Capitol, 

 now deposited in the Patent Office, will convince any one of 

 the great amount of action produced principally by water 

 charged with carbonic acid. Again, every flash of lightning 

 not only generates nitric acid, (which in solution in the rain 

 acts on the marble,) but also by its inductive effects at a 

 distance produces chemical changes along the moist wall, 

 which are at the present time beyond our means of estimat- 

 ing. Also the constant variations of temperature from day 

 to day, and even from hour to hour, give rise to molecular 

 motions which must affect the durability of the material of 

 a building. Recent observations on the pendulum have 

 shown that the Bunker Hill Monument is scarcely for a 

 moment in a state of rest, but is constantl}^ warping and 

 bending under the influence of the ever varying temperature 

 of its different sides. • 



Moreover, as soon as the polished surface of a building is 

 made rough from any of the causes aforementioned, the seeds 

 of minute lichens and mosses, which are constantly floating 

 in the atmosphere, make it a place of repose, and from the 

 growth and decay of the microscopic plants which spring 

 from these, dis-coloration is produced, and disintegration 

 assisted. 



