July :o. 1S7CJ 



NA TURE 



2^1 



iimntemiptedly on to llie end of the bulb, you will suddenly see 

 the black cross (c, d, Fig. 9B) change to a luminous one (e, f), 

 because the background is only faintly phosphorescing, whilst 

 the part which had the black shadow on it retains its full phos- 

 phorescent power. The luminous cross is now dying out. This 

 is a most delicate and venturous experiment, and I am fortunate 

 in having succeeded so well, for it is one that cannot be re- 

 hearsed. After resting for a time the glass seems to partly 

 recover its power of phosphorescing, but it is never so good as it 

 wai; at first. 



We have, therefore, found an important fact c:innected with 



this phosphorescence. Something is projected from the negative 

 pole which has the po«er of hammering away at the glass in 

 front of it, in such a way as to cause it not only to vibrate and 

 become temporarily luminous while the discharge is going on, 

 but to produce an impression upon the glass which is permanent. 

 The explanation which has gradually evolved itself from this 

 series of experiments is this: — The exhaustion in these tubes is 

 S3 high that the dark space, as I showed you at the commence- 

 ment of this lecture, that extended round the negative pole, has 

 widened out till it entirely fills the tube. By great rarefaction 

 the mean free path has become so long that the hits in a given 



time may be disregarded in comparis'.n to the misses, and the 

 average molecule is now allowed to obey its own motions or laws 

 without interference. The mean free palh is in fact comparable 

 to the dimensions of the vessel, and we bave no longer to deal 

 with a continuous portion of matter, as we should were the tubes 

 less highly exhausted, but we must here contemplate the molecules 

 individually. At first this was only a convenient worl>ing hypo- 

 thesis. Long-continued experiment then raised this provisional 

 hypothesis almost to the dignity of a theory, and now the general 

 opinion is that this theory gives a fairly correct explanation of 

 the facts. In these highly exhausted vessels the mean free path 



of the re.iJual molecules of gas is so long that they are able to 

 drive across from the pole to the other side of the tube with 

 comparatively few collisions. The negatively electrified mole- 

 cules of the gaseous residue in the tube therefore dash against 

 anything that is in front, and cast shadows of obstacles just as if 

 they were rays of light. Where they strike the glass they are 

 stopped, and the production of light accompanies this sudden 

 arrest of velocity. 



Other substances besides English, German, and uranian glass, 

 and Becquerel's luminous sulphides are also phosphorescent. 

 I think, without exception, the diamond is the most sensitive 

 substance I have yet met for ready and brilliant phosphorescence. 

 I have here a tube, similar to those already exhibited, containing 

 a mica screen painted with powdered diamond, and when I turn 

 on the coil, the brilliant blue phosphorescence of the diamond 

 can be seen, quite overpowering the green phosphorescence of 

 the glass. Here, again, is a very curious diamond, which I was 

 fortunate enough to meet with a short time ago. By daylight it 

 is green, produced, I fancy, by an internal fluorescence. The 

 diamond is mounted in the centre of this exhausted bulb (Fig. 10), 

 and the negative discharge will be directed on it from below 

 upwards. On darkening the theatre you see the diamond shines 

 with as much light as a candle, phosphorescing of a bright green. 



In this other bulb is a remarkable collection of crystals of 

 diamonds, which have been lent me by Prof. Maskelyne. When 

 I pass the discharge over them I am afraid you will only be able 

 to see a few points of light, but if you will examine them after 

 the lecture, you will see them phosphoresce with a most brilliant 

 series of colours — blue, apricot, red, yellowish green, orange, and 

 pale green. 



Next to the diamond the ruby is one of the most remarkable 

 stones for phosphorescing. In this tube (Fig. II) is a collection 

 of ruby pebbles, for the loan of which I am indebted to my 

 friend Mr. Blogg, of the firm of Blogg and Martin, who placed 

 a small sackful at my disposal. As soon as I turn on the induc- 

 tion spark you will see these rubies shining with a brilliant rich 

 red colour, as if they were glowing hot. Now the ruby is 

 nothing but crystallised alumina with a little colouring-matter, 

 and it became of great interest to ascertain whether the artificial 

 ruby made by M. Fell, of Paris, would glow in the same man- 

 ner. I had simply to make my wants known to M. Feil, and 

 he immediately sent me a box containing artificial rubies and 

 crystals of alumina of all sizes, and from those I have selected 

 the mass in this tube which I now place under the discharge r 

 they phosphoresce of the same rich red colour as the natural 

 ruby. It scarcely matters what colour the ruby is, to begin with. 

 In this tube of natural rubies there are stones of all colours — 

 the deep red ruby and the pale pink ruby. There are some s» 

 pale as to be almost colourless, and some of the highly-prized 

 tint of pigeon's blood ; but in the vacuum under the negative 

 discharge they all phosphoresce with about the same colour. 



As I have just mentioned, the ruby is crystallised alumina. 

 In a paper published twenty years ago by Ed. Becquerel,' I find 

 that he describes the appearance of alumina as glowing with a 

 rich red colour in the phosphoroscope (an instrument by which 

 the duration of phosphorescence in the sunlight can be ex- 

 amined). Here is some chemically pure precipitated alumina 

 which I have prepared in the most careful manner. It has been 

 heated to whiteness, and you see it glows with the rich red 

 colour which is supposed to be characteristic of alumina. The 

 mineral known as corundum is a colourless variety of crystallised 

 alumina. Under the negative discharge in a vacuum, corundum 

 phosphoresces of a rose-pink colour. There is another curious 

 fact in which I think chemists will feel interested. The sapphire 

 is also crystallised alumina, just the same as the ruby. The 

 ruby has a little colouring-matter in it, giving it a red colour ; 

 the sapphire has a colouring-matter which gives it a blue colour, 

 whilst corundum is white. I have here in a tube a very fine 

 crystal of sapphire, and, when I pass the di-charge over it, it 

 gives alternate bands of red and green. The red we can easily 

 identify w ith the glow of alumina ; but what is the green ? If 

 alumina is precipitated and purified as carefully as in the case I 

 have just mentioned, but in a somewhat different manner, it is 

 found to glow with a rich green colour. Here are the two spe- 

 cimens of alumina in tubes, side by side. Chemists would say 

 that there was no difTerence between one and the other ; but I 

 connect them with the induction-coil, and you see that one glows 

 with a bright green colour, whilst the other glow.-! with a rich 

 red colour. Here is a fine specimen of chemically pure alumina, 



" Annales dc Chimie ct de Physique, yi. series, vol. Ivii. p. 50, 1859. 



