The Chemistry, Properties and Tests of Precious Stones

He

e to the passage of heat-rays, but allow them free movement through the substance, whilst, in other cases, no passage of heat is possible, the stones being as opaque to heat as to light. Indeed, the properties of light and heat are in many ways identical, though the test by heat must in all cases give place to that by light, which latter is by far of the greater importance in the judging and isolation of precious stones. It will readily be understood that in the spectrum the outer or extreme light-rays at each side are more or less bent or diverted, but those nearest the centre are comparatively straight, so that, as before remarked, these central rays are taken

the refractive power of heat, applied to the actual stones, and consisting of a fine pencil blowpipe-flame, one line (the one twelfth part of an inch) in length in each case. This list must be taken as approximate, since in many instances the test has been made on one stone only, without possibility of obtaining an average; and as stones vary considerably, the figures may

refracting stones.

t. H

1.436 4.

1.479

l 1.7

ine 1.

2.431 6.

refracting stones.

t. H

5 4.7 singl

1.0 varies

1.635

ryl 1.76

5.1 singl

ndex of 1.63, with a heat index of none

f gypsum is 1.54, but

f amber is 1.51, but h

gures are the average of those obtained from tests made in some cases on several stones of the same kind, and also on isolated specimens. Not only does the power of the stone to conduct heat vary in different stones of the same

faces, angles, lines of symmetry, etc., etc., each one of which gives different results, not only as to value in conductivity, but a result which varies in a curious degree, out of all proportion to the heat applied. In m

placed that they become diathermal, or transparent to heat-rays; whilst, in the other instance, the crystals which so arrange themselves as to be diathermal are, by a sl

at is caused to vary, so do they show the complete gamut between the

o the stones in their several portions was kept constant. It will be seen, therefore