40 NEW YORK STATE MUSEUM 



it can be said to represent the most modern practice, with special 

 reference, of course, to German and continental methods. 



The different physical tests are designed to yield information 

 as to the following properties : specific gravity and weight ; porosity ; 

 absorption ; hardness and toughness ; strength under compressive, 

 transverse, tensile and shearing stresses ; wear or abrasion ; resist- 

 ance to fire ; and durability when exposed to frost, changes of tem- 

 perature and other weathering influences. These will be briefly 

 discussed in their order. 



Specific gravity and weight. The specific gravity of any material 

 is its weight compared with an equal volume of pure water. In 

 the case of solid bodies like rocks that are insoluble in water, the 

 determination is carried out by weighing the samples in air and 

 then finding their weight when suspended in distilled water. The 

 weight in air divided by the loss of weight in water is the specific 

 gravity. The matter, however, is not quite so simple, owing to the 

 fact that rocks are more or less porous and there is some trouble in 

 securing moisture-free samples for the first weighing and complete 

 saturation of the samples for the second. This can be accomplished, 

 however, in the following manner : samples of cubical shape, weigh- 

 ing at least 40 or 50 gramms, are heated in an air bath at no° C. 

 until they show no further loss of moisture, when they are placed in 

 a desiccator and allowed to cool. After weighing, they are immersed 

 in distilled water which at first may be boiled to hasten the expul- 

 sion of air. They should be maintained under water for a period 

 of from three to four days, when they will have reached a con- 

 dition of practically complete saturation. They are then removed 

 from the bath, their outer surfaces rapidly dried with blotting paper 

 and then weighed. It will be found that determinations made in 

 this way are fairly accurate, and there is less opportunity for error 

 through faulty manipulation than by determining the gravity with 

 the use of a picnometer or specific gravity bottle. It gains a 

 further advantage in that the same samples and weights are useful 

 in finding the porosity. 



The weight of stone per cubic foot is usually determined by multi- 

 plying the specific gravity into the weight of a cubic foot of water, 

 which is 62.4 pounds. This is sufficiently accurate for the closely 

 textured rocks, but with porous sandstones a deduction must be 

 made equivalent to the weight of the same rock required to fill 

 the pore space. A more direct method is to weigh a cubic or rec- 



