fF te 
jae 
Stay 
J. Lawrence Smith on Emery. 363 
by repeated weighing the loss sustained bythe glass is reduced “ae a 
to a few milligrammes, The total loss in the glass is then noted, > 
and when all the ‘specimens of emery are submitted to this opera- 
tion under'the same circumstances, we get an exact idea of their 
relative hardness. = ¢ 
The blue sapphire of Ceylon was pulverized and experimented 
with in:this.way; it furnished me‘with a unit of comparison by 
which to compare the results obtained. This operation is long — 
but certain, and for the harder varieties of emery it is necessary 
to repeat the rubbing six or séven times and it requires nearly two 
hours for completion, => 
The results that I have obtained are interesting and have fur- 
nished me with the means of forming conelusions that I could 
or Tt. 
not have otherwise-come at. & 
Glass and dgate-have uot been chosen for this experiment with- 
pieces of agate, with two pieces of glass, and with metal and 
“glass. The agates were found too hard, as they crushed the 
emery without producing hardly any abrasive effect; the-others 
were found not to crush the emery sufficiently, making the ex- 
timent tedious and long. With the glassand agate we have a 
ard substance which crushes the emery, and in a certain space of 
time reduces it to such an impalpable state that it has no longer 
any sensible effect on the glass, and on the other hand, the glass 
is soft enough to lose during this time sufficient of its substance 
to allow of accurate comparative results. In the employment of 
this method in the arts, it would not be necessary to go to the 
sapphire for a standard of comparison; any good emery would 
answer the purpose quite as well. a 
t must be understood that this method of coming at the abra- 
sive effects of emery does not furnish the mineralogical harduess 
of this substance, by which we understand the harduess of any 
individual particle, as evinced by its effect on a substance of less 
hardness,.without regard to the molecular structure of the mineral. 
Two minerals possessing the same hardness but differing in strne- 
ture, one being friable, and the other resisting, will be found very 
different in their abrasive effects; for instance, break a piece of 
quartz in two, subject one of the pieces to a white heat, and after 
cooling, compare the two by rubbing the point against some hard 
substance ; both will be found to scratch equally well: then try 
the two ina state of powder, by rubbing them between two 
pieces of glass that have been weighed, and the difference of their 
abrasive effects will be found very great; because, the one snb- 
Jected to the fire is exceedingly friable, and becomes readily 
crushed to an impalpable powder. This fact is eminently true 
With reference to emery, many specimens of which containing 
the same amount of corundum differ somewhat in their effective 
“e 
