50 H. BURCHARTZ. 
used for testing Portland Cement, the grains of which 
pass the sieve of 60 meshes and are retained on the sieve 
with 120 meshes per sq. cm. 
The dried specimens of material (a) size 7 x 7°1 em. 
(2°8 x 2°8 inches)=50 sq. cm. or 7°75 sq. inches area, are 
weighed before and after the exposure to the sand blast; 
also the weight of volume (b), that is the weight is deter- 
mined (in grammes of the unit of volume of the material; 
including interstices r (in gem.) 
Loss of weight in grammes 
Weight of volume r in grammes 
gives figure expressing the resistance of the material to 
abrasion, and may be used in comparing the value of 
different materials. 
The value of, 
The sand blast apparatus is built by Alfred Gutmann, 
Actiengesellschaft in Ottensen near Hamburg, Germany). 
(a) The specimens are either cut out of the materials 
by means of diamond saws, or formed in moulds if the 
material is made out of a plastic mass like mortar. 
(b) The weight of volume is not to be confounded with 
specific gravity, that is the weight (in grammes) of the 
unit of volume of the material without interstices (in ccm.) 
Weight of volume r 
Specific gravity s 
The value of = ¢L 
The difference :—1 —d =u; the contents of the inter- 
stices, or the porosity. 
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