In that water they knew the unknown metal was concealed, but vast quantities of it had to be evaporated before a residue could be obtained sufficiently large to enable ordinary chemistry to grapple with the metal.  They, however, hunted it down, and it now stands among chemical substances as the metal Rubidium.  They subsequently discovered a second metal, which they called Caesium.  Thus, having first placed spectrum analysis on a sure foundation, they demonstrated its capacity as an agent of discovery.  Soon afterwards Mr. Crookes, pursuing the same method, discovered the bright green band of Thallium, and obtained the salts of the metal which yielded it.  The metal itself was first isolated in ingots by M. Lamy, a French chemist.

All this relates to chemical discovery upon earth, where the materials are in our own hands.  But it was soon shown how spectrum analysis might be applied to the investigation of the sun and stars; and this result was reached through the solution of a problem which had been long an enigma to natural philosophers.  The scope and conquest of this problem we must now endeavour to comprehend.  A spectrum is pure in which the colours do not overlap each other.  We purify the spectrum by making our beam narrow, and by augmenting the number of our prisms.  When a pure spectrum of the sun has been obtained in this way, it is found to be furrowed by innumerable dark lines.  Four of them were first seen by Dr. Wollaston, but they were afterwards multiplied and measured by Fraunhofer with such masterly skill, that they are now universally known as Fraunhofer’s lines.  To give an explanation of these lines was, as I have said, a problem which long challenged the attention of philosophers, and to Professor Kirchhoff belongs the honour of having first conquered this problem.

(The positions of the principal lines, lettered according to Fraunhofer, are shown in the annexed sketch (fig. 55) of the solar spectrum.  A is supposed to stand near the extreme red, and J near the extreme violet.)

[Illustration:  Fig. 55.]

The brief memoir of two pages, in which this immortal discovery is recorded, was communicated to the Berlin Academy on October 27, 1859.  Fraunhofer had remarked in the spectrum of a candle flame two bright lines, which coincide accurately, as to position, with the double dark line D of the solar spectrum.  These bright lines are produced with particular intensity by the yellow flame derived from a mixture of salt and alcohol.  They are in fact the lines of sodium vapour.  Kirchhoff produced a spectrum by permitting the sunlight to enter his telescope by a slit and prism, and in front of the slit he placed the yellow sodium flame.  As long as the spectrum remained feeble, there always appeared two bright lines, derived from the flame, in the place of the two dark lines D of the spectrum.  In this case, such absorption as the flame exerted upon the sunlight was more than atoned for by the radiation from the flame.  When, however, the solar spectrum was rendered sufficiently intense, the bright bands vanished, and the two dark Fraunhofer lines appeared with much greater sharpness and distinctness than when the flame was not employed.