I have here a thin glass bulb containing a mixture of equal volumes of two gases, chlorine and hydrogen. These gases when exposed to a bright light combine together, and form hydrochloric acid gas. If I were to throw this bulb out into the sunlight, so rapid would be the combination, and so great the consequent evolution of heat and sudden expansion, that this little bulb would instantly be shattered into a thousand fragments. Almost as sudden an effect will be produced if I simply burn

a bit of magnesium wire in the neighbourhood of the bulb (Fig. 7); it explodes with a pretty loud report, the bulb is shattered, and the gases have been combined by virtue of the blue rays contained in this kind of light. In order to convince you that the red rays cannot produce the action which the blue ones are able to effect, I here burn a piece of phosphorus, first in a red globe full of oxygen gas, when, as you see, an

C

intense light is emitted wholly without action on our little bulb of sensitive gas. Next I burn a similar piece of phosphorus in oxygen contained in this blue globe, and as soon as the intensity of the light attains its maximum, the bulb explodes with a loud report. I will next show you in another way that it is the blue rays which thus act chemically. This lantern (Fig. 8) contains panes of different coloured glass,-here a white one, there a yellow one; here a red one, there a blue one. I am going to put another of these little bulbs filled with chlorine and hydrogen in the inside of this lantern, and then I will produce, not by magnesium wire, but by another means, a very bright blue light, a light which contains these chemically active rays in great quantity. I will first allow this blue light to shine upon the bulb through the red pane of glass. Here I produce a very bright flame, by throwing some carbon disulphide into a tall cylinder full of nitric oxide gas, and igniting the mixture. There you have the bright flash, but you have noticed no explosion of the bulb, for all the chemical rays have been held back: filtered off by this red glass, they cannot pass through; and the consequence is, there has been no action on my bulb. will now allow another of these flashes of light to pass through the blue glass, which being of course transparent to the blue rays my little bulb will be shattered into a fine powder, as you observe. Here then we have ascertained by experiment that the blue rays act chemically, whilst the red rays produce heating effects.

I

This sensitive mixture of chlorine and hydrogen, which, as you have seen, explodes when the chemical activity of the light is great, may be used as a most delicate means of measuring the amount of light of a

less intensity. The combination of the gases then occurs slowly, and may be rendered evident by allowing the hydrochloric acid thus formed to be absorbed by water, when the consequent diminution in bulk of the gas accurately represents the chemical action effected.

The varying intensity of the chemically active rays in different parts of the solar spectrum has been carefully measured by means of this sensitive mixture of chlorine

and hydrogen gases.1 The accompanying figure (Fig. 9) exhibits the chemical action effected by the various portions of the spectrum on the sensitive mixture for one particular zenith distance of the sun. The lines marked with the letters of the alphabet from A to w, at the bottom of the figure, represent the fixed dark lines which exist in the solar spectrum, of which I shall have much to say in the subsequent lectures. They serve as landmarks by which to ascertain the position of any given

1 See Appendix C for description of method.

point in the spectrum. The greatest amount of chemical action is noticed between the line in the indigo marked G, and that in the violet marked н. In the direction of the red end of the spectrum, the action becomes imperceptible about D, in the orange (the maximum of visible illumination); whilst towards the other end of the spectrum the action was found to extend as far as the line marked U, or to a greater distance beyond the line H in the violet than the total length of the ordinary visible spectrum. For other chemical substances, such as the salts of silver, which are capable of undergoing decomposition when exposed to the light, different curves of sensitiveness are obtained. All these bodies are acted upon by the blue rays, but some have their maximum action at one part, some at another part of the spectrum.

This fact of the greater chemical activity of the more refrangible rays may again be illustrated by showing that I can photograph with these blue rays, whereas I fail to produce the same effect with the red rays. I will coat a plate with collodion, and then darken the room, with the exception of this yellow monochromatic flame, produced by the volatilization of soda salts, which is incapable of acting chemically, and with which we may work without at all affecting our photographic plate. Now I have coated a plate with collodion, and sensitized it in the silver bath. I shall next expose this to the action of the light of the spectrum of the electric lamp. Let me first show you that I have here (Fig. 10) a negative photograph, of which I am about to take a print by means of the blue rays of the electric lamp. You will observe that there are two figures upon the negative, one marked V and the other marked R: these letters being intended to signify Violet and Red. The one figure

marked V, I propose to place in front of my sensitized plate in the blue or violet ray, and the one marked R I shall open in the red ray, and I hope to be able to produce a chemical effect on that portion of my sensitized plate which has been exposed to the blue, whilst we shall get no corresponding effect on the portion exposed to the red ray. I next place my plate with its face downwards on the negative; we now start our electric lamp, using a small spectrum in order to have

the action rather more distinct. I then expose half my plate in the red rays for about twenty seconds, and afterwards expose the other half, with the V upon it, for about the same length of time, to the violet light. I will now develop and wash the photograph, and throw the image produced on the screen, when you will observe (Fig. 11) a very marked difference between the two halves, the one showing that no action has been produced