ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 267 



" The qualitative identification of carbon dioxide in the cavities of 

 a mounted thin section of a mineral may be determined, at least with 

 probability, after some experience, through various optical appear- 

 ances and physical characteristics which have been often described. 

 It is usually effected with certainty and ease, through the rapid and 

 enormous expansion and ultimate disappearance, either of the liquid 

 or of the gaseous bubble on the application 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 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, e. 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 (those of Nachet, Beale, Fuess, 

 Schultze, Chevalier, Dujardin, Eansom, Polaillon, Eanvier, and 

 Vogelsang). In place of all these, a simple and inexpensive apparatus 

 may be substituted, 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 

 (Fig. 51) (tinned iron is liable to rust), of length sufficient to project 



a few centimetres on either side of the stage of the Microscope em- 

 ployed ; the one I use being 23 cm. in length, 4 cm, in width, and 3 cm. 

 in depth. This is laid across the stage, separated from the metal by 

 thin plates of cork c c, and is heated by a short wax taj)er (night- 

 light) underneath either extremity. The slide s may rest upon the 

 bottom, guarded from the metal by little rubber bands r r beneath its 

 ends, and wedged firmly by a little wooden wedge w beneath the 

 horizontal thermometer bulb & ; or a thermometer with a ring-shaped 

 bulb may be inserted, upon which the slide may rest directly, firmly 

 attached by one or two slender rubber bands. The thermometer 

 should be of guaranteed accuracy, with wide degrees, subdivided if 

 possible, with a range which need not much exceed 20° to 32° C. 

 The preparation is then covered by any pure and clear water, prefer- 

 ably filtered (distilled is unnecessary), to a depth of about 2 cm. A 

 circular aperture in the bottom of the box, 18 mm. in diameter, is 

 covered with glass attached by cement, and through this the light is 

 thrown up from the mirror. The cavity to be examined is then care- 



