TESTING BUILDING MATERIALS. 307 



The commission 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 re- 

 sistance to crushing, tests of much importance, as indicating the cohe- 

 sive force of the particles of the stone, and its capacity 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 expansion 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^, because a coarsely crystal- 

 lized stone may ai)parently absorb a small nuantity 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 commission. 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 absorption 

 in these portions of the surface. 



The marble chosen for the Capitol is a dolomite, or, in other words, 

 is composed of carbonate of lime and magnesia in nearly atomic pro- 

 portions. It was analyzed by Dr. Torrey of New York, and Dr. G-enth 

 of Philadelphia. According to the analysis of the former it consists 

 in hundredths parts of — 



Carbonate of lime 54.621 



Carbonate of magnesia 43.932 



Carbonate of protoxide of iron 365 



Carbonate of protoxide of manganese (a trace) mica .472 

 Water and loss 610 



The marble is obtained from a quarry in the southeasterly part of 

 the town of Lee, in the State of Massachusetts, and belongs to the 

 great deposit of primitive limestone which abounds in that part of the 

 district. It Is generally white, with occasional blue veins. The struc- 

 ture is fine grained. Under the microscope it exhibits fine crystals of 

 colorless mica, and occasionally also small particles of bisulphuret of 

 iron. ^ Its specific gravity is 2.8620; its weight 178.87 lbs. per cubic 

 foot ; it absorbs .103 parts of an ounce per cubic inch, and its porosity 

 Is great in proportion to its power of resistance to pressure. It sus- 

 tains 23.917 lbs. to the square inch. It not only absorbs water by 

 capillary attraction, but in common with other marble suffers the dif- 

 fusion of gases to take place through its substance. Dr. Torrey found 

 that hydrogen and other gases, separated from each other by slices of 

 the mineral, diffuse themselves with considerable rapidity through the 

 partition. 



This marble, soon after the workmen commenced' placing it in the 

 the walls, exhibited a discoloration of a brownish hue, no trace ot 

 which appeared so long as the blocks remained exposed to the air in 

 the stonecutter's yard. A variety of suggestions and experiments 

 were made in regard to the cause of this remarkable phenomenon, 



