26 APPARATUS AND METHODS. 



experiment, date, time of day ; the external conditions 

 (temperature, light-intensity, barometer-reading, etc.) ; the 

 precautions which seem necessary, and the sources of error 

 which may spoil the results. 



Always make " control " or " check " experiments, using 

 the same form of apparatus, set up at the same time, but 

 with one or other of the conditions different, e.g. in dark- 

 ness instead of light ; with the plants omitted ; with killed 

 instead of living plants ; with plants in different stages of 

 growth. Also make " repeat " experiments, using different 



Elants under similar conditions or the same plants at dif- 

 jrent times of year or day, etc. 



If your experiments do not succeed, try again ; if they 

 give discordant results, try to account for these and to 

 think out a method for a repeat experiment under different 

 conditions, with special precautions, or for making a new 

 experiment altogether. In drawing conclusions, try to 

 distinguish between probability and actual proof. 



