CH. l] 



EFFECT OF HEAT. 



13 



should be spilt on the floor of the box, so that the atmo- 

 sphere surrounding the object may be damp. A thermo- 

 meter passes through a hole in the lid or, as we find more 

 convenient, through a cork fitting one of the lateral 

 openings. The glass slip on which the object is mounted 

 should be separated from the stage of the microscope by a 

 perforated plate of cork, so that the object may assume 

 the temperature of the air, rather than that of the micro- 

 scope, — although these two temperatures will after a time 

 be nearly identical. 



The hot-box may be conveniently supported on wooden 

 blocks and heated by a gas flame. As the warming of a 

 considerable mass of water is a slow process it is advisable 

 to fill the box with water 10° C. above the room tempera- 

 ture. Notice the accelerating effect of warmth, and record 

 the temperature at which the circulation (1) becomes 

 slower (about 42° C.) ; (2) stops altogether (about 46° C). 



(17) Velten's method. 



A simpler and quicker plan is that of Velten', which, 



Fig. 3. Exp. 17. 

 1 Flora, 1876, p. 177, also F. Darwin, Q. Journal Blicroscopical 

 Science, N.S. Vol. xvii. p. 245. Of. Pfeffer, Zeitschr. fiir wiss. Mikro- 

 skopie, 1890. 



