ZOOLOGY AND BOTANY, MIOKOSCOPY, ETC. 269 



scope, and readily visible without motion of tlie head. Each degree 

 of the column should be about a cm. in length, and subdivided to 

 tenths. 



Lastly, a pointed glass tube, with flexible rubber connection for 

 blowing, and a wire support s, to receive both this and the thermo- 

 meter, attached to the metal plate. 



. The latter is laid upon the stage of the Microscope, separated by 

 thin plates of cork or a perforated piece of pasteboard; the tank, 

 supplied with about 40 cc. of water, is placed over the central aper- 

 ture a, and a taper beneath an extremity of one arm of the plate, and 

 the apparatus is then ready for use in the way already described, the 

 water of the tank being heated by conduction through the metal plate. 

 The section of the mineral is best mounted upon a very thin slide, 

 45 mm. by 26 mm., and this is guarded as before with rubber bands, 

 and held down by one or two little brass weights. Only a single 

 taper is necessary for the low temperature required in the examination 

 of carbon dioxide cavities, and even with this a temperature of 43° C. 

 may be obtained in the bath within a few minutes. The disappearance 

 of the bubble may be completed in less than five minutes, the taper 

 being removed as soon as the rising column approaches within 2 or 3 

 degrees of the critical point, roughly determined by a previous trial. 

 If two tapers are used, the temperature of the water may be raised to 

 55° in about 20 minutes, or even much higher, by the use of Bunsen 

 gas burners. In summer the temperature of the atmosphere alone 

 may be sufficient, especially if assisted merely by the current of warm 

 breath, to obliterate the gas bubble. Its return may be readily caused, 

 in a warm atmosphere, by adding from time to time a few drops of 

 cool water to the bath, while the eye remains at the eye-piece, and a 

 steady current of air is blown through the glass tube. Mounted 

 slides used for such experiments must be labelled by writing with a 

 diamond, or the paper label may be rendered waterproof by being 

 coated successively with weak size and any transparent varnish, such 

 as copal or shellac. 



From these experiments it may be inferred that with this appa- 

 ratus, which may be called the immersion warm bath, it matters little 

 for most purposes what liquid, stand, or objective is employed ; that 

 water is preferable to glycerine, from its greater mobility, convenience, 

 and lack of cost ; that its bulk is immaterial, so long as the bulb of 

 the thermometer is covered; that it is decidedly advantageous to 

 immerse the anterior lens of every objective in the bath, to avoid the 

 annoying interference with observation produced by the vibration of 

 the surface, and by the necessity for repeated refocussing, when the 

 objective is above the surface of the liquid ; that careful determination 

 on minute cavities, with high powers, carried on slowly to enable the 

 preparation, objective, and thermometer to assume the same tempera- 

 ture, may be as accurate as any others ; and that there is no difficulty 

 in obtaining satisfactorily the two determinations within ten minutes 

 to an approximation of about one-twentieth of a degree. 



The descriptions of this method, and of these forms of apparatus, 

 have been given in the more detail, inasmuch as they may be of 



