CHAP. XII. 1. 1. IN LEAVES DURING VEQETATiON. 163 



The surface of the leaf was occasionally moistened 

 with water, with doubtful results as to the effect 

 being increased. If the leaf had been separated 

 from the plant for any time before the circuit was 

 formed, no effect occurred. 



As several of the experiments were carried on in 

 the open air, one or two circumstances occurred 

 which it was necessary to guard against. If the 

 weather was at all boisterous, it became utterly im- 

 possible to continue the experiments, the slightest 

 breeze being sufficient to shake the instrument. To 

 obviate this difficulty, the galvanometer was firmly 

 fixed upon a heavy block of wood, and sheltered. 

 I was, however, perplexed by another circumstance. 

 Working during a fine and calm day, it occasionally 

 happened, that, just upon the point of completing 

 a circuit, the needle would inove to the extent of 10° 

 or 15°, or more, without any apparent cause, the 

 needle up to that time having been perfectly steady, 

 and the circuit not yet completed. After some time 

 it was noticed to occur just after a cloud had passed 

 over the sun, and it was thought to be due to a 

 slight breeze that might be then produced ; but I am 

 now disposed to consider ii as owing to the heating 

 effect of the sun's rays upon the glass shade of the 

 instrument creating a motion of the air within the 

 shade. Some facts bearing upon this question have 

 been noticed by Prof. Txndall'' in reference to some 

 experiments of Dr. Goodman. 



' Philosophical Magazine, Feb. 1853. 



