GENERAL CONDITIONS OF PLANT-LIFE. 



Convenient contrivances for observing the action of particular higher or lower tem- 

 peratures on plants or parts of plants of considerable size are easily arranged ^ It is 

 more difficult to expose microscopic objects to a particular higher or lower temperature 

 in such a manner that it can easily and certainly be observed, and that the temperature 

 of the object is also that indicated by the thermometer, or nearly so. All these require- 

 ments are fulfilled by the very cheap heating apparatus for the microscope represented 

 in Fig. 445. Since I have net only made great use myself of this apparatus for three 



I'IG. 445. — Heatintj apparatus for the microscope. 



years, but have also recommended it to others, a description is the more in place here 

 as it is well adapted for demonstrations in lecture rooms. 



The size of the heating apparatus must vary with that of the microscope ; mine is 

 constructed for one of Hartnack's ordinary instruments. The box is nearly cubical, and 

 has double walls of sheet-zinc at the bottom and sides, enclosing a space 25 mm. thick, 

 which is filled with water through the hole /. It is quite open above ; but in the front 

 side-wall is an opening /, which is closed by a glass plate well fitted but not other- 

 wise fixed. This window is sufficiently large, and is so placed that it allows enough light 

 to fciU on the mirror of the microscope which stands in the box. The height of the 



^ See Sachs, Handb. der Exp.-Phys. pp. 64, 66. 



