MICRO-SPECTROSCOPE INVESTIGATIONS.

LETTER FROM PROFESSOR CHURCH.

THE Editor has received the following interesting letter from Professor Church.

"Have you tried the experiment with chloride of cobalt, which I mentioned to you? If you take the saturated cold solution of this salt it will give the spectrum roughly sketched in Fig. 1, a thinner film of the same solution, heated (on a glass slide with thin cover) over the candle or lamp gives the spectrum drawn in Fig. 2. You will notice two black bands, I had almost said lines, in the red. As might be predicted from the change of colour on heating, the solution is afterwards much more transparent to rays beyond D. The chloride of copper and nickel also give very interesting results.

But I think you will be most pleased with the experiment I have now to relate. I have worked lately on the spectra of pleochroic minerals and salts. Among the minerals recently examined were several fine specimens of the true zircon or jargoon, a silicate of zirconia. These gave a beautiful and most characteristic system of seven dark bands quite different from those belonging to any other substance yet examined. They are roughly sketched in the following figure. Zircons as colour

less as common glass shew these bands as well, perhaps better, than those possessed of colour. They are to be observed with zircons which have been ignited as well as with those still in their natural condition. But some zircons show the phenomenon better than others, this difference not being due apparently to the colour of the stone or the thickness through which the light traverses. I am not quite sure, but I incline to think that those zircons which have come from some localities shew the bands better than those from others. Several Expailly specimens scarcely exhibit anything of this kind; all

The Figure alluded to shews the red darkened, the orange light, and a broad dark band commencing to the right of the yellow and extending beyond the line F, the remainder of the spectrum is cloudy.

Fig. 2 shows the narrow black bands in the red, modified tints replacing the broad dark band of Fig. 1, the blue coming out clear. The experiment is a very beautiful one.

those from Ceylon and Norway show the bands well. From this observation I am induced to hazard the conjecture that it may be, after all, the presence of Swanberg's norium which determines the difference. You are aware that the orange jacinth, a variety of zircon, is very precious, and that the essonite of cinnamon-garnet is constantly sold for it. Curiously enough, the cinnamon-garnet, or essonite, (a lime-garnet), has no conspicuous dark absorption-bands at all, and so the spectroscope may be brought to bear upon the discrimination of these two stones. We have thus a much more ready process than that of taking the density of the specimens. The limegarnet is of comparatively small value. The iron-garnet of different shades (carbuncle, almondine, etc.) gives a beautiful and very characteristic spectrum with several intensely deep absorption bands.

I write these particulars of my experiments at once, for I thought you might like to make a little paragraph about them for the readers of the INTELLECTUAL OBSERVER.

I ought to add that the absorption bands of zircon resemble those of didymium, discovered by Gladstone, in their sharpness and in their being produced by the passage of light through a colourless medium. Silica, the other constituent of zircon, gives no bands.

RESULTS OF METEOROLOGICAL OBSERVATIONS MADE AT THE KEW OBSERVATORY.

*To obtain the Barometric pressure at the sea-level these numbers must be increased by ⚫037 inch.