hydrogen. It is seen that this action attains many maxima, of which the largest lies by GH to H, and the next at I, and also that the action diminishes much more regularly and rapidly towards the red than towards the violet end of the spectrum.

The sun, when it was employed for these experiments, was 35° 13' removed from the zenith. If the atmosphere were throughout of the density corresponding to 0.76 m. and 0° C., the perpendicular height which, during our experiment, it would have possessed, is

The depth of atmosphere through which the rays had to pass in this experiment was, however,

We have stated in one of our previous communications,1 that the solar rays which at different hours of the day pass through the same column of chlorine are altered in a very different manner. This shows that rays of different chemical activity are absorbed in very different ways by the air. The above results are therefore only applicable for sunlight which has passed through a column of air, measured at 0·76 m. and 0° C. of 9,647 metres in thickness. For rays which have to pass through a column of air of a different length from this, the chemical action of the various constituents of the spectrum must be different. The order and degree in which the chemical rays are absorbed, may be obtained by repeating the observations according to the above method from hour to hour during a whole day. Such a series of experiments we have unfortunately as yet been unable to execute, owing to the variability of the weather in our latitudes. One very imperfect series of observations we can, however, quote, and they suffice to show that the relation between the chemical action of the spectral colours is perceptibly altered when the thickness of air through which the rays pass changes from 9,647 to 10,735 metres.

1 Phil. Trans. 1857, p. 617, &c.

These experiments were likewise made on August 14th, 1857, in the short space of time from 9h. 44m. to 10h. 19m. A.M., and gave the following numbers reduced to the zenith distance (42° 46'), corresponding to 10h. Om. A.M. They were, however, made with a bundle of rays of a different thickness from the former experiments, and therefore cannot be compared with those.

From this it is seen that the relation of the chemical action of the spectrum from the line E to the line H undergoes a considerable alteration when the rays have to pass through a column of air 10,735 metres in height instead of 9,647 metres.

An extended series of measurements of the chemical action of the several portions of the solar spectrum under various conditions of atmospheric extinction may prove of great interest, if, as we can now scarcely doubt, the solar spots appear at regular intervals, and our sun belongs to the class of fixed stars of variable illuminating power. It is possible that such observations, made during the presence and during the absence of the solar spots, may give rise to some unlooked-for relations concerning the singular phenomena occurring on the sun's surface. Whether, however, the atmospheric extinction can ever be determined with sufficient accuracy to render visible the alteration in the light which probably occurs with the spots, is a question which can only be decided by a series of experimental investigations which must extend far beyond the scope of any single observer.

LECTURE II.

Continuous Spectrum of Incandescent Solids.-Effect of Increase of Heat. Broken Spectrum of glowing Gas.-Application to Chemical Analysis.-Spectra of the Elementary Bodies.-Construction of Spectroscopes.-Means of obtaining Substances in the state of glowing Gas.-Examination of the Spectra of Coloured Flames. Spectra of the Metals of the Alkalies and Alkaline Earths. Mapping Spectra, according to Bunsen; according to Wave-lengths.-Delicacy of the Spectrum Analytical Method and its application to Physiological Research.

APPENDIX A.-Description of the Spectrum Reactions of the Salts of the Alkalies and Alkaline Earths.

APPENDIX B.-Bunsen and Kirchhoff on the Mode of using a Spectroscope.

APPENDIX C.-Bunsen on a Method of mapping Spectra.

In the last lecture I pointed out to you some of the chief properties of the light with which we are now, I am glad to say, illumined-the light of the sun. I explained that the white sunlight can be divided up into a large number of different coloured rays by means of the prism; that these differently refrangible rays possess different properties; that we find the heating rays chiefly situated at the red end, or in the least refrangible part. I showed that we could separate out by certain means the light rays from the less refrangible ultra-red rays, and obtain at the dark focus of these rays the phenomena of incandescence and of combustion, showing that these rays, which do not affect the eye, are capable when

E

brought together of producing ignition. We also saw that at and beyond the other end, the blue end, of the spectrum we have the rays termed the chemically active rays, and that these rays are capable of effecting chemical change.

We proceed to-day in the examination of the action of heat upon terrestrial matter in so far as it evolves light. The question may very properly be asked, "What has all this to do with chemical analysis?" It might be said, "It is true you have pointed out the difference between the various parts of the solar spectrum; but how is this connected with the analysis which we expect to be told about-with the method by means of which chemical substances may be detected or examined with a degree of accuracy beyond anything that has hitherto been attained?" In order to enable you to answer this question, let us begin by examining the action of heat upon terrestrial matter, and, in the first place, upon solid bodies. I have here the means of heating a long piece of platinum wire, first of all to redness, and by diminishing its length I shall be able to increase the temperature of the wire gradually until I raise it to the melting-point of platinum. The first thing we observe when a solid body, such as this wire, is heated, is that it becomes redhot; and that as we increase the temperature, the light which it gives off increases in refrangibility, so that it ends by emitting light of every degree of refrangibility. I cannot show you on the screen the spectrum which this heated wire yields, simply because the intensity of light which it emits is insufficient for the purpose; but if I were to allow the light to fall into my eye through a prism, I should see that the red rays become first visible, and that then a gradual increase in the refrangibility of

the light occurs, and that successively yellow, green, blue, and violet rays will be emitted as the temperature is increased up to a white heat, when all the rays of light are given off.

I will endeavour to render this fact visible to you in a rougher way by heating the wire gradually up to whiteness, and allowing the light to pass through these coloured glasses placed between you and the wire. At first, when it is red-hot, the glowing wire will be visible only through the red glass, none of the rays being able to pass through the blue glass; or, in other words, there is no blue light given off when the temperature is increased, blue rays begin to be given off, and these can pass through the blue glass, as you now plainly see when I raise the temperature of the wire. Here I can increase the temperature of the wire until we get at a point at which I have no doubt you will be able to see that the blue rays are emitted; and if I continue this and go on until the wire becomes intensely white-hot, you will see it through this blue glass perfectly well.

Such then is the action of increased temperature upon solid bodies. If I had taken any other substance which I could have heated in the same way, I should have produced the same effect: for it has been found that all solid and liquid substances act in this same way with regard to increase of heat; they all begin to be visibly hot at the same temperature, and the spectrum thus produced is in every case a continuous one.1 I may remind you that this is the case by again throwing on the screen the spectrum of the white-hot carbon points

1 This law was discovered by Draper (Phil. Mag. 1847). The only known exception to this law is glowing solid Erbia, whose spectrum exhibits bright lines; see Appendix F to Lecture IV.