212 [Assembly 



But it slionld not be forgotten that all the granite of a quarry may 

 not be of tlie high quality desired ; and in this rock, as well as in any 

 other, though not usually to the same degree, there will be w^ste and 

 refuse material. Though generally more free from iron pyrites than 

 the other rocks, yet this mineral does occur in all the granites, and 

 there is rarely a building erected that does not show its presence ; but 

 in all the quarries examined, from which building-material is obtained, 

 this mineral occurs only in scattered and inconsiderable amounts. 



In those granites, where the crystalline mixture consists of fine or 

 moderately coai'se grains of the different substances intimately min- 

 gled throughout the mass, we may count upon a durable building 

 material, and one subject to a less degree of change from atmospheric 

 agencies than any other stone in our country. 



VII. 



Modes of Determintng the Character and Strength of Build- 

 ing Stones. 



In the erection of all public structures, or those of any considerable 

 magnitude, the strength and durability of the material is of the first 

 importance, and that which should receive the most careful attention. 

 In large and heavy structures the strength of the material is of more 

 importance than in ordinary ones, which never approach a test of the 

 strength or power of resistance of the material composing them. Even 

 with all the experience we have had, and the experiments that have 

 been made, there seems to be no settled opinion of or knowledge 

 among engineers regarding the real strength of the various kinds of 

 stones, either in regard to their direct resistance of pressure or their 

 lateral strength. According to the report of Prof. Henry, the com- 

 missioners appointed to test the stone preparatory to the erection of 

 the extension of the United States Capitol, found that the practice 

 heretofore adopted for testing the strength or resistance to pressure 

 was very defective, and the results unsatisfactory. If the result thus ob- 

 tained he admitted, and of which there can be no doubt, the statements 

 heretofore recorded on these points, and the tables compiled from the 

 exiDeriments made, are to be regarded with many grains of allowance 

 in favor of the stone tested. While the instruments employed by 

 Rennie and others were defective, the plan of placing the. block of 

 stone to be tested between steel plates with a sheet of lead intervening, 

 in order to equalize the pressure from irregularity of the surface of 

 the stone, or want of parallelism in the opposite faces, gave very im- 

 perfect results. 



In experiments reported by Prof. Henry, we have the example of a 

 cube of marble placed between steel plates, with lead intervening, giv- 

 ing way at a pressure of 30,000 pounds; while another block of pre- 

 cisely similar character placed directly in contact with the steel plates, 

 sustained a pressure of 60,000 pounds. "This interesting fact was 

 verified in a series of experiments embracing samples of nearly all the 

 marbles under trial, and in no case did a single exception occur to 

 vary the result. The explanation of this remarkable phenomenon, 

 now that the fact is known, is not difficult. The stone tends to give 

 way by bulging out in the center of the four perpendicular faces, and 



