Chemistry and Physics. 261 
add a few words. In general terms, the spectrum has already 
been described. The particular point to which attention is neces- 
sary here is that at least two of the lines in the spectrum of 
helium, seen with a wide dispersion prism, are coincident with 
two of the argon lines. These occur in the red, and comprise one 
of each of the two pairs of characteristic argon lines. This 
observation has been frequently repeated, using for the purpose 
spectroscopes of different dispersive power, and throwing into 
the field both spectra at the same time, with an exceedingly nar- 
row slit; and we may say that if not absolutely identical, the lines 
are so near that it is not possible with the means at our disposal 
to recognise any difference in position. But the relative brilliancy 
is by no means the same. One of the argon lines, rather faint, is 
coincident with the prominent red of the helium spectrum, and one 
of the strong red argon lines is coincident with a faint red line in 
the helium spectrum. 
Besides these two, there is a line in the orange-red, which though 
perhaps not identical, yet is very close.. This line is faint in 
helium, but moderately strong in argon. It is much more easily 
visible with helium in the “negative glow” than in the capillary 
tube. 
It may also be of interest to state that, according to Runge’s 
observation, the brilliant yellow line of helium is undoubtedly a 
doublet. This was frequently observed by us with a grating of 
14,000 lines to the inch in the spectrum of the third order. But 
it must also be noted that one of the lines is very faint; the 
other, more refrangible, is immensely brighter. The distance, 
judged by eye, appears to be about =th part of that between 
the lines D, and D, of sodium. Accurate information on this 
last point may be looked for from Mr. Crookes, Mr. Lockyer, and 
from many others who are interested in the probable occurrence 
of this element in the sun.* 
General Conclusions.—It cannot be doubted that a close anal- 
ogy exists between argon and helium. Both resist sparking with 
oxygen in presence of caustic soda; both are unattacked by red- 
hot magnesium; and if we draw the usual inference from the 
ratio between their specific heats at constant volume and at con- 
stant pressure, both are monatomic gases. These properties 
undoubtedly place them in the same chemical class, and differen- 
tiate them from all known elements. 
Although opinion is divided on the precise significance of the 
ratio of specific heats, 1°66, it appears to be most probable that 
in all cases, as in that of mercury, this ratio points to the mona- 
tomicity of the molecule. If we assume this provisionally, it 
follows that the atomic weight of helium is identical with its 
molecular weight. The molecular weight is twice the density, 
for the molecular weights of gases are compared with the atomic 
weight of hydrogen, taken as unity; hence the atomic weight of 
* Prof. Hale and Dr. Huggins have recently observed that the solar line Ds is 
also a doublet. (W. R., July 20.) ; 
