sunbeam. If I clap my hands, or if I shake my coat, or if I knock this dusty book, I think you will observe that this flame becomes yellow, and this not because it is the hand or coat of a chemist, but simply because the dust which everybody carries about with him is mixed with sodium compounds. When I place in the colourless flame this piece of platinum wire, which has been lying on the table for a few minutes since I heated it red-hot, you see there is sodium in it; there, we have for one moment a glimpse of a yellow flame. If I heat the wire in the flame, the sodium salts will all volatilize, and the yellow tinge will quite disappear; but if I now draw the wire once through my fingers, you observe the sodium flame will on heating the wire again appear. If I draw it through my mouth and heat it again, it will be evident that the saliva contains a very considerable quantity of sodium salts. Let me leave the wire exposed here, tied round this rod, so that the end does not touch anything, for ten minutes or a quarter of an hour; I shall then obtain the sodium reaction again, even if the wire be now perfectly clean. This is because sodium salts pervade the atmosphere, and some particles of sodium dust flying about in the air of the room settle on the wire, and show their presence in the flame.

I hope in the next lecture to consider the history of the subject, and to point out to you that this constant reaction of sodium puzzled the old observers very much. They thought this reaction must be due to the presence of water, for there was no other substance which was so commonly diffused; and it is only recently that this yellow reaction has been recognized as being due to this metal, sodium.

To refer for a moment to the distribution of lithium

compounds we must remember that this substance, giving the beautiful red flame which you saw just now and the spectrum exhibiting the one bright red line, was until lately only known to exist in three or in four comparatively rare minerals. The moment, however, we come to examine substances by the method of spectrum analysis, we find that the brilliant red line, which is characteristic of the presence of lithium, occurs very frequently. And why, then, was not the red flame noticed before? Because when the light was examined by means of the eye alone, the red-coloured flame was masked by the presence of soda salts, and other substances affecting the flame, so that the red tint produced by the small quantity of lithium was unseen. But when we examine the flame with the prism, then all these lines range themselves into due order, no one interfering with the other. The presence of lithium may be thus easily detected, though it may be mixed with ten thousand times its bulk of sodium compounds, because, as you see by reference to this chart, the sodium line occurs. in a different position to the lithium line, according to the differences in their refrangibilities. We now learn that this supposed rare substance is found to be most widely distributed,-not, it is true, in very large quantities, but still that it is one of the most widely diffused of the elementary bodies. Lithium not only occurs in very many minerals, but also in the juice of plants, in the ashes of the grape, in tea, coffee, and even in milk; in human blood, and in muscular tissue. It has also been found by Dr. Bence Jones in many varieties of fruit and vegetables, in all the different kinds of wine, in ale and porter, and in bread. So who can tell what part this hitherto rare substance may not play even in the

animal economy? It has been also found in meteoric stones, in the water of the Atlantic Ocean, as well as in that of most mineral springs and many rivers. It is present in the ashes of tobacco, and, if we hold the end of a cigar in the colourless flame, we may always notice the red lithium line when the light is examined with a spectroscope. Dr. W. Allen Miller has lately found lithium in very large quantities in the water of a spring in the Wheal Clifford Mine in Cornwall. This water contains 26 grains of lithium chloride in one gallon, and the spring flows at such a rate as to pour forth 800 lbs. of this salt every twenty-four hours!

Here we have a table showing the great delicacy of the methods of spectrum analysis :

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1. Sodium. 3000000 part of a milligramme, or 180000000 part of a grain, of soda can easily be detected. Soda is always present in the air. All bodies exposed to the air show, when heated, the yellow soda line. If a book be dusted near the flame, the soda reaction will be seen. 2. Lithium. 100000 part of a milligramme, or 6000000 part of a grain, can be easily detected. Lithium was formerly only known to exist in four minerals: it is now found by spectrum analysis to be one of the most widely distributed elements. It exists in almost all rocks, in sea and river (Thames) water, in the ashes of most plants, in milk, human blood, and muscular tissue.

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3. Strontium. 100000 of a milligramme, or 1000000 of a grain, of strontia is easily detected. Strontia has been shown to exist in very many limestones of different geological ages.

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4. Calcium. 100000 of a milligramme, or grain, of lime can be easily detected.

1 Chem. News, x. 181.

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5. Cæsium and Rubidium. These new alkaline metals were discovered by Bunsen in the mineral waters of Baden and Dürkheim. Forty tons of mineral water yielded 200 grains of the salts of the new metals.

6. Thallium. A new metal discovered by Mr. Crookes in 1861, distinguished by the splendid green line which its spectrum exhibits. It is found in iron pyrites, and resembles lead in its properties.

7. Indium. Discovered in zinc blende by Professors Reich and Richter: found in very minute quantities. It is distinguished by one blue and one indigo coloured band seen in its spectrum.

I will now endeavour to illustrate, by means of the electric lamp, the fact that all these bodies give off coloured lights, and that each of these coloured lights is of a peculiar kind; and I would wish first to show you that when we bring a small fraction of a grain of common salt, chloride of sodium, on to the lower carbon of the lamp, we obtain a distinct yellow band which was not seen before, for previously, you will remember, we had a perfectly continuous spectrum. This yellow band is due to the presence of sodium. You will probably see that there are other bands present as well as the sodium band, because it is impossible, owing to the delicacy of these reactions, to obtain any carbon which is perfectly free from other chemical salts, and the small impurities which exist in the carbon come out as evidence against us on the screen; yet I think you will see that we have the sodium line more distinctly visible than anything else.

No other metal but sodium gives this yellow band; still I must beg you to understand that this rough representation is not exactly that which you would see if you