CEYPTOGAMIC 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. 
Hxperiments 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 ight 1s 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. 
