32 
HAROLD’S DISCUSSIONS. 
shows its own peculiar line or lines and no other, and 
no two substances give the same lines. 
The spectroscope is very delicate in its tests. 
Caesium was discovered in a mineral water that con¬ 
tained so little of it that forty-two tons of the water 
had to be boiled down to get a quarter of an ounce of 
the caesium. The instrument shows the presence of 
even one-five-millionth part of sodium in a com¬ 
pound. 
The sun’s rays and the refiected light of the plan¬ 
ets, and even of the moon, have been examined, and 
their spectra show that these bodies contain most, if 
not all, of the substances found on the earth. This 
looks as if they were a ‘‘ slice of the same loaf.” 
Thus has science imprisoned wandering rays of 
light from even the most distant stars, which rays 
started hither long before Alexander marshaled his 
hosts to conquer the world, and has made them give 
up the secret of what substances their orb is com- 
