396 



E. A. 8CHAPER. 



the inflow tube into the stage, while the colder fluid passes 

 into the reservoir through the lower tube to supply its place. 



Fig. 2. Fig. 3. 



Fig. 2. — Ideal section of apparatus, b. Central chamber in stage. 

 c. Vertical reservoir heated by small gas flame below, and enclosing bulb of 

 mercurial regulator, d. e, e. Connecting tube of india rubber. The arrows 

 show the direction of the gas in the regulator and of the currents of water 

 in the heating apparatus respectively. 



Fig. 3. — Gas regulator, about natural size. j,j. Upper part of bulb 

 (rather small in proportion), a. Tube with side openings, to the lower of 

 which is attached the steel collar h, in which works the screw g. f. Small 

 steel tube with slit, i, i. Mercury. The arrows show the direction of the gas. 



The chamber in the centre of the stage is, when in use, 

 closed below by a circular cover-glass, which is placed on a 

 rim made for its reception, and previously oiled ; above, it is 

 covered by the cover-glass, on which the object is placed; it 

 communicates, however, with the exterior by means of a 

 small lateral tube which passes through the body of the 

 stage, but is entirely shut off from the surrounding fluid. 

 The bulb of a small thermometer may be introduced through 

 this lateral tube whenever it is wished to ascertain the tem- 

 perature of the central chamber (see fig. 1, c, in horizontal 

 section). This method of measuring the temperature has a 

 considerable advantage over that in which the thermometer- 

 bulb encircles the wall of the chamber and lies in the 

 surrounding fluid, for this last is always very perceptibly 

 warmer than the interior of the chamber. It has the dis- 

 advantage that observations cannot be made while the ther- 

 mometer is in situ, but this can be met by slightly withdrawing 

 the instrument into the lateral tube. Indeed, ordinarily it 

 will be sufficient, when the desired temperature is attained 



