162 Transactions of the Aiiertcan Instttxtte. 



toward this sodium flame, we shall see only one image of the slit ; 

 and that image will fall in a definite position in the field of the 

 instrument. However, after we repeat the experiment we shall 

 always find that single image in that single position. Whenever, 

 therefore, we see a single image or band in that definite position, we 

 know that we have sodium vapor as the source of the light. 



Let us try another experiment in illustration of the same point. 

 I will now introduce into the flame another platinum wire, on the 

 end of which is melted a bit of another metallic element, which we 

 call lithium. It is not the pure metallic lithium, but a compound of 

 lithium ; and you see that the lithium vapor is the source of light. 

 The gas with which the lithium is combined volatilizes in the flame 

 and fills the flame with its vaj^or ; but the light emitted by the dense 

 metallic vapor is so much more intense than the light which emanates 

 from the gas, that the color you see is solely the color of the lithium 

 vapor, which emits a beautiful red light. If we place our colored 

 card in that red light you see that it appears of a very pure red 

 color ; because here the red rays are reflected from the colored card. 

 If we examine this light with a spectroscope we see a band in this 

 position [indicating the lithium line on the chart] ; and whenever 

 with our spectroscope we see a red band in- that position we know 

 that lithium is the source of the light, [Applause.] 



There is still another method of experimenting on these vapors, 

 which is used in fireworks. That method consists in mixing the 

 material on which we wish to experiment with sulphur and nitre, 

 and then igniting the mixture. The sulphur burns with a pale flame, 

 and the metallic vapor distributed throughout the mass of this flame 

 emits its peculiar light. For example, we have here a mixture in 

 which the nitre has been mixed with a preparation of another metallic 

 element we call barium ; and the green light emitted is chiefly the 

 light of the barium vapor. Again, I have here a similar mixture, in 

 which, instead of barium, strontium has been used, another metallic 

 element ; and this red light is chiefly the light which comes from the 

 Btrontium vapor, which is volatilized by the heat and distributed 

 through the sulphur flame. This method of experimenting gives us 

 essentially the same result as the other. In order to show you that 

 this is the case, I have prepared here a similar mixture of sulphur, 

 containing common salt; and while that is burning I will return to 

 the flame of the Bunsen burner and the platinum wire with the salt 

 Tipon it ; 60 that you have here the flames of the salt burning in 



