BUILDIKG STOKES. 185 



Qualitative tesls.] 



included in ascertaining the individual credits of the stones, but another 

 element, not included in the physical tests, has been introduced, viz., facility 

 of dressing, which plainly is governed by the hardness or the toughness of 

 the rock ; the former depending on its content of free silica, and the latter on 

 the crystalline texture, or the nature of the cement in the case of sediment- 

 ary rocks. This operates disastrously on the crystalline rocks, and wholly 

 to the advantage of the softest rocks. Yet there is good reason for allow- 

 ing this, though this quality has, perhaps, less to do with the actual value 

 of the stones as material for building, since an excess of hardness (or crush- 

 ing resistance) at least up to 25,000 pounds per square inch, which is equally 

 unimportant in ordinary construction, has been allowed to act in its full 

 capacity in favor of the crystallines. The variation of texture induced by 

 lamination, or accompanied by it, is a very important element in sediment- 

 ary rocks. 



In forming any judgment of the comparative value of these building- 

 stones for individual markets, other elements should be included along with 

 these physical tests, such as ease of quarrying, accessibility of the quarry, 

 facility of transportation and permanence of the supply. 



THE METHODS OF THE PHYSICAL TESTS. 



The detail of the methods of the physical tests, as given by Trof. Dodge, is as follows : 

 Determination of specific gravity. This was executed by the usual method for solid bodies, 

 with the use of an analytical balance. The specimens were nearly of the same size and 

 approximately cubical, having been split out by hammer and chisel. They measured from an inch 

 to an inch and a half on a side, and weighed on an average about 50 grammes. They were in an 

 air-dried state, having been allowed to lie on a table in a warm and dry room for several weeks 

 before their specific gravity was determined. They were then severally weighed. They were then 

 immersed in water and allowed to remain about twenty-four hours, when all evolution of air bub- 

 bles had ceased. They were then weighed in water, suspended by a thread. The weight in air was 

 divided by the loss of weight in water, and the result was taken as the specific gravity. 



Determination of the absorption of moisture from a damp atmosphere. The samples of stone 

 were placed in the cells of a hot-water bath for several days, to expel their hygroscopic moisture. 

 They were then allowed to cool in desiccators, over sulphuric acid, and were weighed. They were 

 then placed upon a set of glass shelves standing in a pan of water, and a tight cylinder was inserted 

 oyer the shelves, the mouth of the cylinder being s?aled by the water, after the manner of a gas 

 holder. The apparatus remained thus in a room whose temperature was pretty uniform, from 60 

 to 70 Fahrenheit, for seven weeks, the water being replenished from time to time, so as to main- 

 tain a constant closure of the cylinder. Then the stones were removed to bell-jars in which they 

 were supported over water, and thus taken to the balance and weighed. The stones submitted to 

 this test were somewhat larger than the pieces used for making the determination of specific gra- 

 vity. They had an average weight of about 70 grammes. They were roughly shaped. The min- 

 imum absorption of moisture .03 per cent, of the weight of the stone, is so small in amount as to 

 be practically nothing. The maximum, 3.94 per cent of the weight of the stone, seems quite con- 

 siderable. It seems probable that, in the atmosphere saturated with moisture in which they were 

 kept for seven weeks, some of the stones absorbed all the moisture they were capable of taking up, 



