23 
large. In many instances it were better to first sharpen the tool 
on a coarse whetstone before using the finer variety. As both the 
rapidity and fineness of abrasion depend on the size of the cutting 
erains, the effectiveness of two stones will be as the cubes of the 
diameters of these grains. Hence a stone with grains of a diam- 
eter of one will abrade steel eight times as fast as a stone having 
grains with a diameter only half that size, but on the other hand 
the work of the latter stone will be eight times as fine. 
2. In examining the effect of the form and mineral com- 
position of the grit grain we notice that in the fine grained stones 
the grains should be uniform in size, for should there be large 
grains among the smaller ones it would be impossible to obtain a 
fine edge. The grains should also be angular in form otherwise 
they will not scratch, it is this rounded character of the grains 
which causes some stones to glaze after being used for a time. 
The amount of foreign matter among the grains also influences 
the abrading qualities, for if it separates the grains too much the 
scratches will be too far apart for effective work. The hardness 
of the material itself also effects its usefulness, but as silica is the 
abrading substance in almost all whetstones, it really makes the 
form of the silica the last influence. In sandstones the silica is 
in rough irregular grains, in schists in irregular massive grains 
scratching by means of angles and points, while in the novacul- 
ites it is in minute angular grains. The varieties of silica also 
vary slightly in hardness depending on the manner in which the 
particles are joined together. 
Finally, in considering the effects of structure we find that 
the particles may be consolidated simply by pressure, either with 
or without the présence of earthy material; or they may be act- 
ually cemented together, with or without the presence of earthy 
matter; or consolidation may be due to both of these causes. In 
the Labrador stone we have an excellent example of a stone in 
which the cementation is the important factor although it has also 
been subjected to pressure. It contains very little earthy matter. 
An iron cement binds the particles of silica together, and so 
strong is this cement that unless the stone is used hard it has a 
tendency to glaze. 
