930 Gencral Notes. | November, 
ties of a mounted thin section of a mineral, may be determined, 
at least with probability, after some experience, through various 
optical appearances and physical characteristics which have been 
often described. It is usually effected with certainty and ease, 
through the rapid and enormous expansion and ultimate disap- 
pearance, either of the liquid or of the gaseous bubble, on the appli- 
cation of a gentle heat for a few seconds, such as that of a cigar, 
the heated end of a rod, or jet of hot air, or even a jet of the 
warm breath conveyed through a flexible rubber tube. When 
the slide and the thin section are thin, even the heat (37° C.) of 
the tip of one’s finger, applied for a few seconds to the bottom of 
the slide without removal from the stage of the microscope, May 
be sufficient to produce the characteristic phenomena, ¢. g., the 
contraction and disappearance of a bubble whose size is relatively 
small to that of the liquid in which it floats. 
“For the determination of the temperature of disappearance of 
the bubble, which may vary from 20° to 32° C., several forms of 
stage heating apparatus may be employed. * * * In place 
of all these, a simple and inexpensive apparatus may be substi- 
tuted, consisting of a miniature water bath in which are immersed 
the entire section and slide, the bulb of the thermometer, and the 
nose of the objective. It consists of a box of tinned copper 
(tinned iron is liable to rust), of length sufficient to project a few 
centimeters on either side of the stage of the microscope em- 
ployed ; the one I use being twenty-three centimeters in length, 
4 centimeters in width, and three centimeters in depth. This 1s 
laid across the stage of the microscope, separated from the metal 
by thin plates of cork, and is heated by a short wax taper (night- 
light) underneath either extremity. The slide may rest upon the 
The preparation is then covered yA 
any pure and clear water, preferably filtered (distilled is unneces: 
sary), to a depth of about two centimeters. A circular apertur 
in the bottom of the box, eighteen millimeters in diameter, 
covered with glass attached by cement, and through this : 
light is thrown up from the mirror. The cavity to be examine’ 
is then carefully adjusted and focussed, a taper is lit, and the ey 
-Temains at the eye-piece until the critical point is reached, © 
_ glass tube with its point terminating just below the edge 0 4 
slide, is connected with the mouth during the experiment by t 
_ small rubber tube. As the temperature slowly rises, 2 constan 
a Current of small bubbles of the warm breath (whose temperature, 
