THE MOLECULAR FORCES IN PLANTS. 203 



of metal. In it slides a tube (supply tube), which is put in con- 

 nection with the gas supply. The regulator is placed in the water 

 space of the thermostat. In many cases, however, it is advisable 

 to let the regulator project into the interior of the thermostat. 

 For temperatures up to 40 C. it is filled with mercury and 

 ether; for temperatures from 40-70 C., with mercury and 

 alcohol. 



In heating up the thermostat, we pull the supply tube as far as 

 possible out of the regulator. As the temperature rises, the ether 

 or alcohol vapour raises the mercury in the regulator, and when 

 the desired temperature is reached, the supply tube is pushed in 

 till the triangular opening at its lower end is just covered by the 

 mercury. The extinction of the flame is prevented by the pre- 

 sence in the supply tube of a fine opening, through which gas can 

 still pass to the burner. 



The variations in atmospheric pressure, whose magnitude can 

 easily be determined by means of a barometer, are not without 

 influence on the action of the regulator. Account must be taken 

 of this in using the regulator, if it is desired to keep the tempera- 

 ture in the apparatus constant to a fraction of a degree Centigrade. 

 This is done in the manner described by Bohrbeck in a short 

 paper, which if desired can be obtained from him with the regu- 

 lator. 



In cases in which it is only required to keep the temperature 

 for some time approximately constant, we may also conveniently 

 employ the following apparatus, which moreover, in absence of a 

 hothouse, serves well in winter, especially in cold weather, for 

 rapidly developing seedlings, etc. The thermostat, which rests on 

 a tirm stand, consists of a large double- walled box made of strong 

 sheet zinc. The box is about -60 cm. deep, and the same in length 

 and breadth. The space between the double walls of the appara- 

 tus may be 3 or 4 cm. This is filled with water, which we admit 

 through an opening in the upper part of the box, while in the 

 lower part of it there is a tap, from which to run off the water 

 when necessary. Through an opening in the double-walled cover 

 of the box passes a thermometer, It remains to be mentioned, 

 that the front of the box consists of a double door. The 

 apparatus is heated by means of a gas flame placed beneath 

 it. 



Thermostats should naturally be placed, if at all possible, in 

 as little as possible subject to changes of temperature. 



rooms 



