Stability.—­These substances vary greatly in stability under influences difficult to appreciate.  I have two specimens of the gold yellow substance, C, both made in December, 1886, with the same proportions, under the same conditions.  One has passed to dazzling white, normal silver, without falling to powder, or undergoing disaggregation of any sort; the fragments have retained their shape, simply changing to a pure frosted white, remaining apparently as solid as before; the other is unchanged, and still shows its deep yellow color and golden luster.  Another specimen made within a few months and supposed to be permanent has changed to brown.  Complete exclusion of air and light is certainly favorable to permanence.

Physical Condition.—­The brittleness of the substances B and C, the facility with which they can be reduced to the finest powder, makes a striking point of difference between allotropic and normal silver.  It is probable that normal silver, precipitated in fine powder and set aside moist to dry gradually, may cohere into brittle lumps, but these would be mere aggregations of discontinuous material.  With allotropic silver the case is very different, the particles dry in optical contact with each other, the surfaces are brilliant, and the material evidently continuous.  That this should be brittle indicates a totally different state of molecular constitution from that of normal silver.

Specific Gravities.—­The allotropic forms of silver show a lower specific gravity than that of normal silver.

In determining the specific gravities it was found essential to keep the sp. gr. bottle after placing the material in it for some hours under the bell of an air pump.  Films of air attach themselves obstinately to the surfaces, and escape but slowly even in vacuo.

Taken with this precaution, the blue substance, B, gave specific gravity 9.58, and the yellow substance, C, specific gravity 8.51.  The specific gravity of normal silver, after melting, was found by G. Rose to be 10.5.  That of finely divided silver obtained by precipitation is stated to be 10.62.[1]

   [Footnote 1:  Watts’ Dict., orig. ed., v. 277.]

I believe these determinations to be exact for the specimens employed.  But the condition of aggregation may not improbably vary somewhat in different specimens.  It seems, however, clear that these forms of silver have a lower specific gravity than the normal, and this is what would be expected.

Chestnut Hill, Philadelphia, May, 1889.

—­Amer.  Jour. of Science.

* * * * *

TURPENTINE AND ITS PRODUCTS.[1]

   [Footnote 1:  Read at a meeting of the Liverpool Chemists’
   Association.]

By EDWARD DAVIES, F.C.S., F.I.C.