594 Original Articles. | Oct., 
mentary substance has its peculiar lines, and probably no line of any 
one element is exactly coincident with that of any other. We are 
thus furnished in the spectroscope with an exceedingly delicate test of 
the presence of any substance. We have only to pass the electric 
spark through the body which we wish to analyze, and the sudden 
flash perceived is in reality a small portion of the vapour of that body 
in a state of intense ignition. If we analyze this by the spectroscope, 
we shall obtain lines that will enable us to determine the substance 
employed. So delicate is this method, that we can detect with ease ° 
the presence of quantities of a sodium salt less than saoom of a grain 
in weight. Nor is it valuable only for substances with which we are 
familiar, but it has also been the means of our discovering new 
elements. The metals Cesium and Rubidium were thus discovered 
by Bunsen ; Thallium, by our countryman Crookes; and Indium, by 
Messrs. Reich and Richter. 
So much for ignited vapours. On the other hand, the spectra of 
incandescent opaque solid or liquid bodies, or of all bodies of very 
great thickness, are continuous throughout, and consist of a bright 
space, varying in colour from one portion to another, but without 
lines. We have thus three varieties of spectra. 
First. Solar and stellar spectra, exhibiting a bright ground inter- 
spersed with dark lines. 
Secondly. Those of heated vapours, consisting of bright lines on a 
dark ground. 
Thirdly. Those of opaque, solid, and liquid bodies, consisting of a 
continuous brightness without lines. 
loess 
We must now request our readers to accompany us to quite a 
different part of our subject, although in the end we hope to trace its 
connection with that which has preceded. 
