A NEW CONSTITUENT OF THE ATMOSPHERE. 223 
(f) A second determination was carried out, without further circulation. 
162°843 cub. centims. measured at 769:0 millims. (corr.) pressure, and 16:00° C., 
weighed 0°2757 grm. Hence, 
Weight of 1 litre at 0° and 760 millims. . . .  1°7713 germs. 
Density compared with hydrogen(O = 16). . . 19°82 . 
(g.) After various experiments had been made with the same sample of gas, it was 
again circulated until all absorption ceased. A vacuum-tube was filled with it, and 
showed no trace of nitrogen. 
The density was again determined :— 
162°843 cub. centims. measured at 750 millims. (corr.) pressure, and at 15°62° C., 
weighed 0°26915 orm. 
Weight of 1 litre at 0° and 760 millims. . . . 1'7707 grms. 
Density compared with hydrogen (O= 16). . . 19°82 - 
These comprise all the determinations of density made. It should be stated that 
there was some uncertainty discovered later about the weight of the vacuous globe in 
(b) and (c). Rejecting these weighings, the mean of (e), (f), and (g) is 19°88. The 
density may be taken as 19°9, with approximate accuracy. 
It is better to leave these results without comment at this point, and to return 
to them later. 
11. Spectrum of Argon. 
Vacuum tubes were filled with argon prepared by means of magnesium at various 
stages in this work, and an examination of these tubes has been undertaken by 
Mr. Crookes, to whom we wish to express our cordial thanks for his kindness in 
affording us helpful information with regard to its spectrum. The first tube was 
filled with the early preparation of density 19°09, which obviously contained some 
nitrogen. A photograph of the spectrum was taken, and compared with a photograph 
of the spectrum of nitrogen, and it was at once evident that a spectrum different from 
that of nitrogen had been registered. 
Since that time many other samples have been examined. 
The spectrum of argon, seen in a vacuum tube of about 3 millims. pressure, consists 
of a great number of lines, distributed over almost the whole visible field. Two lines 
are specially characteristic ; they are less refrangible than the red lines of hydrogen or 
lithium, and serve well to identify the gas when examined in this way. Mr. Crooxss, 
who gives a full account of the spectrum in a separate communication, has kindly 
furnished us with the accurate wave-lengths of these lines as well as of some others 
next to be described ; they are respectively 696°56 and 705°64 X 107° millim. 
Besides these red lines, a bright yellow line, more refrangible than the sodium line, 
