46 H. BURCHARTZ. 
in a comparatively short time. While it is true that one 
can pave small tracts of a street, etc., with various materials 
and expose them to the trafiic, which would be the best 
method of determining the resisting force against the 
influence of mechanical wear, or of the atmosphere, such 
practical experiments often last several years before the 
question is definitely decided as to which is the most dur- 
able and the cheapest material. No modern community 
can afford to wait such a long time. 
It is most desirable to be able to determine the relative 
values of different materials for paving and similar purposes 
in a comparatively short time. However, the method 
generally used is the abrasion tests proposed by Bausch- 
inger, which consists in grinding specimens of material 
on cast iron discs with corundum, or the materials are 
treated in tumbling cylinders, (rattlers), with or without 
steel balls. Neither of these methods gives a reliable 
result. The small parts of the material under trial are 
separated in the grinding process and increase the wearing 
out; on the other hand, they reduce the effect of the shocks 
in the rattler by filling the interstices of the material. 
Again, as the grinding material itself is used up it is 
impossible to prevent the same grains being used repeatedly, 
thus changing their shape and effect, and with soft elastic 
materials the hard grains of the grinding powder partly 
penetrate the test piece, thus reducing the grinding effect; 
the grains now rub each other instead of the surface of 
the specimen, as this is protected by the grains fixed in it. 
In consequence of the defects of this method of grinding, 
the results obtained do not give a true guide to the 
behaviour of the materials under practical conditions. 
A new method of treating the materials, by which the 
difficulties and errors of the methods now in use are avoided, 
which moreover possesses the great advantage that the 
