Scientific Lectures. 163 



sulphur, and here the same flame produced by salt burning in gae. 

 Therefore, whether we adopt the method used in fireworks, or that of 

 burning in the flame of gas, we have in each case simply the light 

 which comes from the incandescent vapor of the different metals. 

 But the purity of the light is much less in the sulphm- flame, because 

 the sulphur flame itself is far more intense than the gas flame, and 

 tends of course to vitiate the result. "We have still a third method of 

 experimenting, which is to pass a current of electricity through the 

 vapor, which is thus made to shine far more brilliantly than in any 

 other way. This method I shall exhibit to you at the close of 

 my lecture. 



These experiments, then, while they have confirmed the general 

 principles I have laid down, have also illustrated the valuable method 

 of analysis to which I referred at the opening of the lecture. In 

 order to discover the metals which are present in a given substance, 

 we have only to place the material in the flame and examine the 

 light which emanates from it. If we see a yellow band in this posi- 

 tion, we know that there is sodium in the flame. If we see a red 

 band in this position, we know that there is lithium in the flame. If 

 we see the series of green bands represented here, we know that there 

 is barium in the flame. If we see this series of red bands, and espe- 

 cially a beautiful blue band in this position, we know there is strontium 

 in the flame. Moreover, this method of analysis is wonderfully deli- 

 cate, and enables us to distinguish quanties of these several substances 

 which could not have been detected in any other way. If we see a 

 new band which has never been observed before, we know that some 

 new metallic element must be present ; and by following out the 

 indications thus obtained, we have, within the last few years, discov- 

 ered no less than four new metallic elements which have been called 

 rubidium, coesium, thallium and indium, names which are derived 

 in each case from the Greek name of the color of the flame. But 

 more than this ; this method of analysis has led to important conclu- 

 sions in regard to the heavenly bodies. Most of the fixed stars, and 

 our sun, show us pure white light. They are, therefore, incandescent, 

 solid or liquid globes. But we find in the heavens a class of bodies 

 to which we have given the name of nebulae. Many of these nebulae 

 send to us, also, pure white light. They, therefore, must consist of 

 solid or liquid globes. But there is another class of nebulae whieh 

 send to us only colored light, and this light, when analyzed by the 

 spectroscope, shows us certain distinct definite bands. These nebulai 



