78 LIGHT. [CH. IV 



Compare the effect on the plant with the readings of a 

 wet and dry bulb thermometer. 



(96) Light and darkness. 



Select a rounded bushy branch of a tree, and if neces- 

 sary tie in the branches to make the specimen compact. 

 Fit the branch air-tight into a hole in a glass plate resting 

 on a large tripod, the branch will thus be above the plate 

 and may be covered with a tubulated bell-jar, while the 

 apparatus is below and free to be manipulated. Into the 

 tubulure fit a cork through which two glass tubes pass, 

 one of which is connected with an aspirator so that a 

 current of air is drawn through the bell-jar. The other 

 tube is connected with chloride of calcium tubes so that 

 the air in the jar may be dry. Readings of the potometer 

 and of the hygrometer inside the bell-jar should now be 

 taken in varying conditions of the air^. The current 

 aspired must not be too rapid, or the air pressure inside 

 the jar will be less than atmospheric pressure; a small 

 mercury manometer fitted into the cork of the tubulure 

 will show this difference if it arises. If the air inside can 

 be kept at a constant hygrometric condition the effect of 

 alternate light and darkness may be shown as in Kohl's 

 research : the bell-jar is darkened by a cover, which can 

 be removed when darkness is exchanged for light. Kohl's 

 method is an excellent one for getting rid of the difficulty 

 which meets the experimenter in all researches involving 



1 If it is merely desiued to show the effect of dry and damp air in a 

 rough way, a, simple bell-jar may be used which may either cover the 

 plant completely or be removed, or may be propped up to produce an 

 intermediate hygrometric condition. 



