TESTING OF BUILDING MATERIALS BY SAND BLAST APPARATUS. 49 
the different parts, the finer or coarser grain, the uniformity 
or the irregularity of the wearing out, and as a result 
whether the material or its constituents are of equal or 
unequal hardness. It shows the thickness of the coloured 
layer of burned plates, and of the fine grained covering of 
concrete plates, as well as the fibrous structure of the 
various kinds of wood. 
All these effects of the sand blast on various materials 
will be clearly seen from the accompanying photographs. 
These peculiarities and characteristics of materials are not 
shown by the grinding or rattling process. By treating 
materials on a Bauschinger grinding disc a flat surface 
only is obtained. Some results obtained in the Prussian 
Testing Laboratory, mentioned above, on abrasion tests 
made with different materials on the Bauschinger machine, 
and also by means of the sand blast apparatus are given in 
TableI. The data given by these results sufficiently prove 
that the new method described above is most suitable for 
testing road and floor materials, as it gives reliable infor- 
mation on their quality and on their resistance to wear 
and abrasion in practical use. At the same time the test 
under the sand blast gives valuable suggestions as to the 
possibility of protecting building materials. 
The duration of the exposure to the sand blast has been 
fixed at two minutes after many experiments, the steam 
gauge indicating 2 atms. pressure, or 44 Ibs. per sq. inch. 
This short time suffices to get a good summary of the 
structure of the materials tested and their resisting force. 
The sand used in the Prussian testing institute isa natural 
quartz sand of fine and nearly round grains, obtained by 
washing and drying the original sand, and sieving the same 
on a sieve with 120 meshes per sq. c/m., or about 784 
meshes per sq. inch (28 meshes per lineal inch). It is the 
waste of the manufacturing of the German standard sand 
D—July 4, 1906. 
