TESTING BUILDING MATERIALS. 303 



ON THE MODE OF TESTINa BUILDINQ MATERIALS, 



AND AN ACCOUNT OF THE MARBLE USED IN THE EXTENSION OF THE UNITED 



STATES CAPITOL. 



BY PROFESSOR JOSEPH HENRY, 



SECRETART OF THE SMITUSONIAN INSTITUTION. 



[Read before the American Association for the advancement of science.] 



A commission was appointed by the President of tlie United States, 

 in November, 1851, to examine the marbles which were offered for the 

 extension of the United States Capitol, which consisted of Greneral 

 Totten, A. J. Downing, the Commisioner of Patents, the Architect, 

 and myself. Another commission was snbsequently appointed in the 

 early part of the year 1854 to repeat and extend some of the experi- 

 ments, the members of which were General Totten, Professor Bache, 

 Captain Meigs, and myself. 



A part of the results of the first commission were given in a report 

 to the Secretary of the Interior, and a detailed account of the whole 

 of the investigations of these committees will ultimately be presented 

 in full in a report to Congress, and I propose here merely to state 

 some facts of general interest, which may be of importance to those 

 engaged in similar researches. 



Though the art of building has been practised from the earliest 

 times, and constant demands have been made, in every age, for the 

 means of determining the best materials, yet the process of ascertain- 

 ing the strength and durability of stone apjjears to have received but 

 little definite scientific attention ; and the commission, who had never 

 before made this subject a special object of study, were surprised with 

 unforeseen difiiculties at every step of their progress, and came to 

 the conclusion that the processes usually employed for solving these 

 questions are still in a very unsatisfactory state. 



It should be recollected that while the exterior materials of a build- 

 ing are to be exposed for centuries, the conclusioms 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 w© fully 

 estimate the amount of those which are' known. 



The solvent power of water, which even attacks 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, car- 

 bonic acid in solution. The attrition of silicious dusts, when blown 

 against a building, or washed down its sides by rain, is evidently 

 operative in wearing away the surface, though the evanescent portion 



