34 Mr. T. Wedgwood's Experiments and Observations 
changes into a beautiful lilac, which gradually fades away. Fe- 
tid marbles, and some kinds of chalk, give a bright reddish or 
orange light ; pure calcareous earth, a bluish white light ; 
Cornish moorstone emits a fine blue light ; powder of ruby 
gives a beautiful red light, of short continuance. 
The most phosphorescent marble is soft and friable, of a 
coarse crystallized grain, and a fetid odour when rubbed ; 
black and grey marbles are generally more luminous than the 
white. Most of the common white marbles are hard, and of a 
fine grain, and they are not very luminous, nor is their light of 
an orange colour. Different chalks vary as much as different 
marbles, in the intensity and colour of their light, when no 
difference of external structure is perceptible. The most phos- 
phorescent chalk loses the brilliancy and redness of its light 
by being dissolved in an acid, and precipitated by caustic fixed 
vegetable alkali — by being combined with vitriolic or fluor acid 
— by calcination by heat, or being combined with the aerial 
acid in the pellicle formed on the surface of lime-water. Marble 
would probably be affected in a similar manner. The most 
phosphorescent blue fluor gives the same light after being 
united to the vitriolic acid, though gypsum is far less lumi- 
nous than fluor, and its light is colourless. Argil precipitated 
from alum by an alkali, and magnesia, when combined with 
fluor acid, give out the same light as before. 
Bodies emit their light when immersed in boiling acid of 
vitriol, or in boiling oils ; small lumps of fluor or of marble 
make a singular appearance in the acid, as they are moved up 
and down by its action, and rendered brightly luminous by the 
heat : they seem equally luminous in pure, fixed, inflammable, 
or atmospheric air. 
Feldspat, the fetid fluor, and probably all phosphorescent 
