Nature of the so-called Elements. 107 
To sum up, then, the facts regarding calcium, we have first 
of all the H-line differentiated from the others by its almost 
solitary existence in Sirius) We have the K-line differentiated 
from the rest by its birth, so to speak, in a Aquilx, and the 
thickness of its line in the sun, as compared to that in the are. 
We have the blue line differentiated from H and K by its thin- 
ness in the solar spectrum while they are thick, and by its 
thickness in the are while they are thin. We have it again 
differentiated from them by its absence in solar storms in which 
they are almost universally seen, and finally, by its absence 
during eclipses, while the H and K lines have been the bright- 
est seen or photographed. Last stage of all, we have calcium, 
distinguished from its salts by the fact that the blue line is only 
visible when a high temperature is employed, each salt having 
a definite spectrum of its own, in which none of the lines to 
which I have drawn attention appear, so long as the tempera- 
ture is kept below a certain point. 
Tron. 
