490 REPORT— 1889. 



this condition is fulfilled. A globular nebula, the diameter of wLicb is ten times 

 the radius of the orbit of Neptune, and consisting of meteorites 1 cm. in diameter, 

 and of the density of iron, is supposed to have a total mass equal to 0-001 of that 

 of the sun. Its internal constitution is not such as could really exist, in that its 

 density and velocity of agitation are supposed to be uniform. It would probably 

 be possible to extend the method to cases in which these quantities vary. The 

 velocity of agitation is assumed to be O'l km. per second, and the relative velocity 

 of approach 4 km. per second. This description of the nebula will also apply 

 approximately to a part of which the distance from the nucleus is ten times the 

 radius of the orbit of Neptune, except that the velocity of approach will have 

 increased. Provided, then, that comparatively few collisions occur while tbe 

 nebula moves from this distance to the nucleus, no great error will be introduced 

 if the calculations are conducted on the supposition that it has travelled from an 

 infinite distance without internal collision. It is shown that these conditions are 

 amply fulfilled in the case selected, and that at a distance from the nucleus of 

 10' km., and throughout a disk of wliicli the radius is ^ x 10'' km., one collision 

 per second would take place at points about 170 km., or, in round numbers, 100 

 miles apart. If we assume that the ordinary formulae for gases may be applied to 

 the original nebula, this shows an increase in collision frequency per unit volume 

 in the ratio 10"^: 1. On the other hand, it is difficult to suppose that luminosity 

 would be produced unless the collisions took place at points in closer proximity. 

 The assumptions on which the method of calculation is based do not, however, 

 permit of its application to nebuhe, of which the density is much more than ten 

 times greater than that chosen in the example. All, therefore, that the calculation 

 shows is that iu the limiting cases of great rarity, and at the distance from the 

 nucleus considered, we cannot expect a collision iu every second at points much 

 nearer than 100 miles apart. In the case of denser nebulre difl:erent mathematical 

 methods would have to be employed. The enormous increase iu collision frequency 

 is, however, well illustrated by the calculations. In the case considered it is of 

 the order lO'". 



6. He-examination of tJie Spectra of 23 Gas-Vacuuvi, End-o7i Tubes, after six 

 to ten years of Existence and Use.^ By C. PiAZZi Smyth. 



This inquiry began in an attempt to ascertain by refined mensurations whether 

 there was any sensible difference of spectral place for liydrogen lines, Avhen they 

 appeared adventitiously and scantily in tubes of other and very ditJerent gases, or 

 in tubes of nothing but pure and abundant hydrogen by original intention ? 



But after having obtained a negative in every case, the inquirer became more 

 taken up with the changes that had occurred in certain of the tubes subsequent to 

 1880, when he published upon them in the ' Trans. Roy. Soc. Edinburgh.' 



Thus a chlorine tube, of which it was printed in 1880 that it was then still 

 showing its chlorine lines, though fainter, after two years' use ; while carbon bands 

 and hydrogen lines had begun to appear : yet now, in 1889, it has nothing but 

 hydrogen lines, and in great brilliance, to show. 



Again, an iodine tube which had a comparatively large quantity of solid iodine 

 granules introduced into, and sealed up in, its interior eleven j^ears ago ; and 

 showed then a splendid spectrum of 148 measured iodine lines, extending discon- 

 tinuously from red to violet, and had nothing else, save three very faint puny 

 images of the three principal lines of hydrogen — this tube, in 1889, has not a smgle 

 iodine line now left ; but its spectrum, which is brighter than ever, is composed of 

 nothing but hydrogen lines, so that the once solid iodine granules would seem to 

 be partly changed into hydrogen, and partly deposited on the inside of the tube as 

 yellow haze, besides leaving a trifle of loose dust. 



The author also mentions with much satisfaction that a London maker, Mr. 

 Charles Casella, transcended all others by supplying him with one tube of C 0, 

 two of N, and two of 0, which have, through six years of existence and work, 

 shown their respective spectra without a trace of hydrogen. 



' Printed in extenso in the Chemical JVems, Nov. 8, 1889. 



