534 Lord Ray high [April 5, 



As to the spectrum, we have been indebted from the first to 

 Mr. Crookes, and he has been good enough to-night to bring some tubes 

 which he will operate, aud which will show you at all events the 

 light of the electric discharge in argon. I cannot show you the 

 spectrum of argon, for unfortunately the amount of light from a 

 vacuum tube is not sufficient for the projection of its spectrum. 

 Under some circumstances the light is red, and under other circum- 

 stances it is blue. Of course when these lights are examined with 

 the spectroscope — and they have been examined by Mr. Crookes with 

 great care — the differences in the colour of the light translate them- 

 selves into different groups of spectrum lines. We have before us 

 Mr. Crookes' map, showing the two spectra upon a very large 

 scale. The upper is the spectrum of the blue light; the lower 

 is the spectrum of the red light ; and it will be seen that they 

 differ very greatly. Some lines are common to both ; but a great 

 many lines are seen only in the red, and others are seen only in the 

 blue- It is astonishing to notice what trifling changes in the condi- 

 tions of the discharge bring about such extensive alterations in the 

 spectrum. 



One question of great importance, upon which the spectrum 

 throws light is, is the argon derived by the oxygen method really 

 the same as the argon derived by the magnesium method ? By Mr. 

 Crookes' kindness I have had an opportunity of examining the spectra 

 of the two gases side by side, and such examination as I could make 

 revealed no difference whatever in the two spectra, from which, I 

 suppose, we may conclude either that the gases are absolutely the 

 same, or, if they are not the same, that at any late the ingredients by 

 which they differ cannot be present in more than a small proportion 

 in either of them. 



My own observations upon the spectrum have been made princi- 

 pally at atmospheric pressure. In the ordinary process of sparking, 

 the pressure is atmospheric ; and, if we wish to look at the spectrum, 

 we have nothing more to do than to include a jar in the circuit, and 

 to put a direct-vision prism to the eye. At my request, Professor 

 Schuster examined some tubes containing argon at atmospheric 

 pressure prepared by the oxygen method, and I have here a diagram 

 of a characteristic group. He also placed upon the sketch some of 

 the lines of zinc, which were very convenient as directing one exactly 

 where to look. See figure on page 535. 



Within the last few days, Mr. Crookes has charged a radiometer 

 with argon. When held in the light from the electric lamp, the 

 vanes revolve rapidly. Argon is anomalous in many respects, but 

 not, you see, in this. 



Next, as to the density of argon. Professor Ramsay has made 

 numerous and careful observations upon the density of the gas pre- 

 pared by the magnesium method, and he finds a density of about 

 19*9 as compared with hydrogen. Equally satisfactory observations 

 upon the gas derived by the oxygen method have not yet been made, 





