GENERAL CONDITIONS OF PLANT-LIFE. 
All these requirements are fulfilled by the very cheap heating apparatus for the micro- 
scope represented in Fig. 474. Since I have not only made great use myself of this 
apparatus for three 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 
Fig. 474. — Heating apparatus for the microscope. 
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 fall on the mirror of the microscope which stands in the box. The height of the 
box is so arranged that the upper rim of the double wall is on a level with the arm I? of 
the microscope. The opening of the box is closed by a thick cardboard cover d d, in 
which an opening is cut exactly to fit the arm b. By the side of the tube of the micro- 
scope a round hole is cut in the cover through which a closely fitted small thermometer 
t is passed, so that its bulb hangs near the object. The box is painted on the inside 
with black varnish, and a piece of cardboard moistened with water lies beneath the foot 
