ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 467 



vessel from a small flame at the side of the stage. By this means an ac- 

 curate determination of the actual temperature at which a fluid inclusion 

 expands into a gaseous state may be obtained in a few minutes to 0*05 C^. 

 The simplest form of the apparatus consists of three parts, as follows : — 



1. A shallow glass tank, such as may be cut off the bottom of a chemical 

 beaker, of sufficient diameter for the slide to lie within it, just immersed in 

 a thin layer of water, but separated from the bottom by two little blocks of 

 rubber or glass. This tank is placed upon the stage. 



2. A chemical thermometer of sufficient delicacy, with a short bulb, or 

 with a long bulb bent at a right angle. This is inserted in the tank, as 

 nearly upright as possible, and the depth of the water is made just enough 

 to cover the bulb. The length of the scale should be such as to bring the 

 degrees between 27° and 32° near the level of the observer's eye when it is 

 at the eye-piece, to facilitate immediate observation without the delay caused 

 by moving the head. 



3. A piece of small rubber tubing tied to the body of the stand, with 

 the upper end inserted in the observer's mouth, and with the lower end, 

 which terminates in a short piece of glass tubing drawn to a fine aperture, 

 lying in the water on the bottom of the tank. 



An immersion objective may be employed or, if the cavity be large, any 

 objective of lower power may be used, with its front immersed in the water. 

 After the cavity has been brought into sharp focus, a steady but gentle 

 stream of air is blown through the tube, the immersion of the objective 

 preventing interference from the waves on the surface of the agitated 

 water. The cavity is continuously observed, as the bath and the immersed 

 thin section are gradually warmed by the current of the observer's breath, 

 and when the critical point is reached and the liquid contents of the cavity 

 suddenly disappear, a quick observation of the thermometer is made. 



Again, as the bath cools — vp-hich process may in hot weather be 

 hastened by adding carefully a few drops of cool water, with continual 

 agitation by the air current — the original bubble may be observed to leap 

 back into view, and a second observation of the thermometer is taken as a 

 check to the first. 



If a higher temperature be required for other uses of this apparatus, oil 

 or other liquid may be substituted for the water in the bath, and it may 

 be heated by conduction from a taper or lamp burning by the side of the 

 stage, through a stiff slip of copper introduced beneath the glass tank, 

 A small hole, for observation, through this copper slip should be placed 

 immediately over the centre of the aperture of the stage. The apparatus 

 may be further protected from radiation of heat, and more uniform results 

 ensured, by inclosing the tank in a ring of pasteboard or sheet cork, and 

 by inserting plates of cork between the copper plate and the stage. 



Unequal Heating- of Crystal Sections.* — Dr. W. Klein, for studying 

 the alterations of optical characters in crystals, produced by unequal heat- 

 ing, suggests the use of a plate of copper, resting upon one side of the 

 crystal, the other end of the plate being heated in a spirit-lamp. To accele- 

 rate the process, and to obtain the means of rotating the section during 

 heating, it is better to use a pair of copper forceps attached to a wooden 

 ring, so that the points of the forceps in which the section is held come 

 exactly into the centre of the ring ; between the ring and the forceps is a 

 layer of asbestos. The whole is laid upon the stage, and the projecting 

 end of the forceps heated by a spirit-lamp. By this method the crystal is 

 heated on one side on both the upper and lower surfaces. 



* Zeitschr. f. Kiystullogr. u. Mineral., ix. (ISSi) pp. 38-72. 



