CH. l] EFFECT OF HEAT. 17 



felt lining of the lid should be wetted and a little water 

 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 (42 46 C.); (2) stops altogether (about 46- 

 50 C.). 



(17) Veltens method 1 . 



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

 however, should not be used with a valuable microscope. 

 The objective and the preparation are immersed in water 

 contained in a glass dish standing on the stage of the 

 microscope. A siphon, provided with a tap, allows warm 

 water to run into the dish, while a second siphon and tap 



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

 Science, N.S. Vol. xvn. p. 245. Cf. Pfeffer, Zeitschr. fur wiss. Mikro- 

 skopie, 1890. 



D. A. 2 



