CRYPTOGAMIC PLANTS. 97 



lours/' says Professor Harvey, "has not been 

 particularly sought after." 



I cannot say these pertain to phosphoric phe- 

 nomena, but I have cited the above cases on ac- 

 count of their curiosity. They appear rather to 

 belong to the optical phenomenon of interference. 



Experiments made with a view of investiga- 

 ting the physical causes of the phosphorescence 

 of decayed wood, alluded to above as owing to a 

 minute cryptogamic organism, have consisted in 

 placing the luminous wood into different gases, 

 plunging it under water, etc. Bockman has proved 

 that phosphorescent wood is as luminous in pure 

 nitrogen and in a void, as in pure oxygen, and 

 that its light is extinguished even in oxygen gas, 

 if the temperature be rather high; also, that it 

 remains luminous under water. This ingenious 

 experimentalist has remarked that moisture ex- 

 alted, to a remarkable degree, the phosphoric in- 

 tensity of decayed wood, and that it appeared es- 

 sential for its manifestation. 



In general a certain degree of warmth and 

 moisture, combined sometimes with a peculiar 

 electrical state of the atmosphere though this 

 does not always seem essential appear to be the 

 most favourable conditions under which we ob- 

 serve vegetable phosphorescence. 



