34 



J, Henry on testing Building Materials. 



After this important fact was clearly determined, lead and all 

 other interposed substances were discarded, and a method devised 

 by which the upper and lower surfaces of the cube could be 

 ground into perfect parallelism. This consists in the use of a 

 rectangular iron frame, into which a row of six of the specimens 

 could be fastened by a screw at the end. The upper and lower 

 surfaces of this iron frame were wrought into perfect parallelism 

 by the operation of a planing machine. The stones being fas- 

 tened into this, with a small portion of the upper and lower parts 

 projecting, the whole were ground down to a flat surface, until 

 the iron and the face of the cubes were thus brought into a 

 continuous plane. The frame was then turned over, and the op- 

 posite surfaces ground in like manner. Care was of course taken 

 that the surfaces thus reduced to perfect parallelism, in order to 

 receive the action of the machine, were parallel to the natural 

 beds of the stone. 



All the specimens tested were subjected to this process, and in 

 their exposure to pressure were found to give concordant results. 

 The crushing force exhibited in the subjoined table is much 

 greater than that heretofore given for the same material. 



The commission have also determined the specific gravities of 

 the different samples submitted to their examination, and also 

 the quantity of water which each absorbs. 



They consider these determinations, and particularly that of 

 the resistance to crushing, tests of much importance, as indica- 

 ting the cohesive force of the particles of the stone, and its ca- 

 pacity to resist most of the influences before mentioned. 



The amount of water absorbed may be regarded as a measure 

 of the antagonistic force to cohesion, which tends, in the expan- 

 sion of freezing, to disintegrate the surface. In considering, 

 however, the indication of this test, care must be taken to make 

 the comparison between marbles of nearly the same texture, be- 

 cause a coarsely crystallized stone may apparently absord a small 

 quantity of water, while in reality the cement which unites the 

 crystals of the same stone may absorb a much larger quantity. 

 That this may be so was clearly established in the experiments 

 with the coarsely crystallized marbles examined by the commis- 

 sion. When these were submitted to a liquid which slightly 

 tinged the stone, the coloration was more intense around the 

 margin of each crystal, indicating a greater amount of absorp- 

 tion in these portions of the surface. 



The marble which was chosen for the Capitol is a dolomite, 

 or is composed of carbonate of lime and magnesia in nearly 

 atomic proportions. It was analyzed by Dr. Torrey of New 

 York, and Dr. Grenth of Philadelphia. According to the analy- 

 sis of the former, it consists, in hundredth parts, of 



