THE PROPERTIES OF BUILDING STONES, ETC. 347 
macadam but it is thought that such tests will prove valuable 
and important for determining the suitability of stone for steps, 
sidewalks, and flooring. 
The abrading machine that is considered best suited for 
determining the coefficient of wear is that used by the Wis- 
consin Geological Survey, and patterned after the machine used 
by the Massachusetts Highway Commission." 
Specific gravity—The determinations of the specific gravity 
of building stones that have come under my observation have 
been based upon two very different conceptions. According to 
one of these conceptions the specific gravity depends entirely 
upon the mineralogical composition, and is independent of the 
porosity of the stone. According to the other conception the 
pores are considered a part of the stone and the specific gravity 
is computed for the exterior volume. These two methods give 
different results for the same stone, and have been designated 
by Regis Chauvenet as ‘Specific Gravity Proper” and ‘Apparent 
Specific Gravity.” Where the porosity of a stone is less than 
I per cent. the two specific gravities are almost the same. 
But where the porosity is 10 or 25 per cent., they are very 
different. 
In most discussions of building stone tests the principle laid 
down by Professor J. C. Smock? that, the specific gravity of the 
particles or mineral species composing the rock mass, determines 
that of the stone has been followed. 
The practical engineer, however, objects to this method, 
because he cannot compute the weight of the stone per cubic 
foot directly by multiplying by 62.5, the weight of an equal 
volume of water. Several contemporary writers on building 
stones, however, have unfortunately made the mistake of deter- 
mining the ‘specific gravity proper” and then computing the 
weight per cubic foot by,multiplying this directly by 62.5. For 
* For description of this machine see the Report of the Massachusetts Highway 
Commission for 1899, pp. 59, 60. 
? Bulletin of New York State Museum, Vol. II, No. 10, p. 374, PROFESSOR J. C. 
SMOCK. 
