138 JOURNAL OF THE [June, 



from the bottom by two little blocks of rubber or glass. This 

 tank is placed upon the stage of the microscope. 



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 eyepiece, to facilitate immediate observation with- 

 out 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 observ- 

 er's breath, and when the critical point is readied and the liquid 

 contents of the cavity suddenly disappear, a quick observation 

 of the thermometer is made. 



Again, as the bath cools, — which 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, an 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 im- 

 mediately 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 



