1889.] Rubies, Sapphires, Diamonds, and some other Minerals. 293 



contained in the same specimen also behave in the same manner, and 

 become full and suddenly boil at almost absolutely the same temperature, 

 as that figured. Vie need scarcely say that such cavities are extremely 

 rare, and are very remarkable even when merely looked upon as microsco- 

 pical objects, independently of their interest in connexion with phvsics. 

 Fig. 3 is a tubular cavity of more irregular form, and is interesting on ac- 

 count of there being two plates of the sapphire projecting into the cavity 

 so as to nearly divide it into three portions. At the ordinary temperature these 

 partitions prevent the passage of the bubble from one part to the other ; 

 but by breathing on the object through a flexible tube, the slight increase of 

 temperature expands the liquid so as to make the bubble small enough to 

 pass into the next compartment ; and a repetition of the process causes it 

 to pass into that at the other end. Such plates projecting into the cavities 

 are very common ; and it is requisite to pay attention to this fact, since 

 otherwise they might easily be mistaken for crystals of some other substance 

 included in the cavity, which, if they ever occur, must be extremely rare, 

 since no decided case has come under our notice. 



In examining sections of sapphire cut in a plane more or less parallel to 

 the principal axis of the crystal, the double refraction is so strong that two 

 images of every object lying at any depth below the surface are seen, in such 

 a manner as to make them very confused. This may be avoided by using 

 polarized light without an analyzer, and arranging the plane of polarization 

 so as to coincide with one of the axes of the crystal. High powers may 

 then be used with perfect definition ; and they show many small cavities, 

 sometimes of most irregular forms, like fig. 4 ; and very often their sides 

 are so inclined that they totally reflect transmitted light, and appear black 

 and opake. In some specimens most of the cavities have lost their fluid. 



Besides fluid-cavities, there are many small crystals of other minerals 

 included in sapphires, but not so many as in rubies. The most striking 

 are small plate-like crystals, often of triangular form, with one angle very 

 acute, They are very thin, and give the colours of thin plates ; so that 

 when viewed by reflected light they look something like the scales from a 

 butterfly. Seen edgewise, they appear as mere black lines, and are ar- 

 ranged parallel to the three principal planes of the sapphire, as shown by 

 fig. 5. These small crystals and the minute fluid- cavities cause many 

 sapphires to appear milky by reflected, and somewhat brown by trans- 

 mitted light * and being arranged in zones related to the form of the crys- 

 tal, they often show, as it were, lines of growth. 



Rubies. 



Though the ruby and the sapphire are of course essentially the same 

 mineral, yet their structure is in many respects as characteristically different 

 as their colour. The number of the fluid-cavities in rubies is far less, and 

 the larger cavities are very rare, and only contain what appears to be water 

 or a saline aqueous solution, as is shown by the amount of expansion when 



