ISTo. 104.] 201 



testing the strength or power of resistance to pressure, etc. The 

 exposure to freezing and thawing will not only determine its power 

 of resisting the action of the weather, but will determine also whether 

 such foreign ingredients as iron pyrites may exist in the mass. 

 Chemical analysis may be resorted to for the purpose of comparison 

 with specimens of known composition and durability ; but chemical 

 analysis alone cannot determine, without other testing experiments, 

 the strength or power of endurance of the stone. 



In some countries, and in certain localities in our country, the evi- 

 dence obtained from ancient structures is av^ailable in determining 

 the durability of the stone which has been used. Yet it would seem 

 that this information has been of little avail in many places, where 

 the rebuilding of edifices is repeated every century. Experience in 

 many cases does not teach the lesson anticipated ; and when a dilapi- 

 dated structure is pointed out, the argument is made that " these 

 stones were not well selected," or they were obtained '• at the first 

 opening of the quarry, and were not as good as now furnished." 

 And again, as already remarked, there are few cases in which parties 

 are '[3ermitted to select the material without prejudice, the influence 

 of interest, or the absence of important information. Examples are 

 everywhere before us of the improper selection of materials for 

 buildings, and these examples do not deter from their use in the 

 erection of others. When good material is abundant and accessible, 

 it will be used ; in other situations, comparatively few durable struc- 

 tures are likely to be erected. 



VI. 



Genekal Composition and comparative Dueability of Building- 

 Stones. 



All the stones used in building, under whatever name they may 

 be known, are composed of a few essential elementary minerals; 

 these are : 



1. Silica or Quartz ; 



2. Alumina-Clay or Argillaceous Matter ; 



3. Carbonate op Lime; 



4. Carbonate of Magnesia. 



Beyond these, except in crystalline rocks, the presence of other 

 material is almost non-essential to the composition of the stone, often 

 accidental or adventitious, and usually injurious to the integrity of 

 the mass. The ultimate chemical ctnnposition of a stone has little 

 to do, as a general rule, with its character for durability ; nor will a 

 chemical analysis determine the value of a stone for building pur- 

 poses. 



Physical Conditions of the Aggregates of the several named 



substanoes. 



1. The silica or quartz may occur as a mechanical aggregation of 

 [Assem. Doc. No. 104.] 26 



