1895.] Argon, a new Constituent of the Atmosphere. 279 



The value of a, being involved only in the excess of weight above 

 that of oxygen, does not require to be known very accurately. Suffi- 

 ciently concordant analyses by two methods gave a = 0'1845 ; whence 

 for the weight of the gas we get 3' 193, so that, if 2 = 16, the density 

 of the gas would be 19'45. An allowance for residual nitrogen, still 

 visible in the gas before admixture of oxygen, raises this number to 

 19*7, which maybe taken as the density of pure argon resulting from 

 this determination. 



X . Density of Argon prepared by means of Magnesium. 



The density of the original sample of argon prepared has already 

 been mentioned. It was 19 '09 ; and, after sparking with oxygen, it 

 was calculated to be 20'0. The most reliable results of a number of 

 determinations give it as 19'90. The difficulty in accurately deter- 

 mining the density is to make sure that all nitrogen has been 

 removed. The sample of density 19'90 showed no spectrum of nitro- 

 gen when examined in a vacuum tube. It is right, however, to 

 remark that the highest density registered was 2O38. But there is 

 some reason here to distrust the weighing of the vacuous globe. 



XT, Spectrum of Argon. 



The spectrum of argon, seen in a vacuum tube of about 3 mm. 

 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. 

 Crookes, who will give 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, 10~ 6 mm. 



Besides these red lines, a bright yellow line, more refrangible than 

 the sodium line, occurs at 603 '84. A group of five bright green 

 lines occurs next, besides a number of less intensity. Of the group 

 of five, the second, which is perhaps the most brilliant, has the wave- 

 length 561'00. There is next a blue or blue-violet line of wave- 

 length 470'2 ; and last, in the less easily visible part of the spectrum, 

 there are five strong violet lines, of which the fourth, which is the 

 most brilliant, has the wave-length 420*0. 



Unfortunately, the red lines, which are not to be mistaken for 

 those of any other substance, are not easily seen when a jar discharge 

 is passed through argon at atmospheric pressure, unless a large jar 

 and a very powerful current be employed. The spectrum seen 

 under these conditions has been examined by Professor Schuster. 

 The most characteristic lines are perhaps those in the neighbourhood 



