592 



NA TURE 



[April i8, 1895 



sysletn, that if all the dust, or meleorite?, or conglomerations of 

 particles, whatever they may be, are going the same way, there 

 will be a condiiion in which we shall get a minimum of col- 

 lisions, and therefore a minimum of temperature. 



The probability, therefore, is that we are not dealing with 

 gas, but with masses of matter in certain regions of which, in 

 consequence of general action, there is greater luminosity given 

 off by the particles of which the nebula; are composed ; in 



5000 



5005 



5010 



Pb M- 



Nebula. 



ComparisoD 

 lines. 



Fig. 20. — Vormal position of the chief nebular line, according to Keeler. 



other regions where there is less action, we have lower tempera- 

 ture and less light. 



If, as was at 6rst imagined, these nebulae are gases at 

 enormous temperatures, it would be a question of seeing them 

 or not seeing them; there would be no special parts to be 

 picked out at all. But, in the case of those nebula: lo which 

 modern photographic methods have been applied, we find that 



the same scale, in which the nebula that we usually see occupies 

 only a very small portion ; the only difference between the 

 two photographs is that one has been exposed for a very long 

 time to enable us to fix and to study the very dim reproduction 

 of certain parts of it, whilst the first one was exposed only for 

 a short time, in order that we might dwell effectively on that 

 part only of the nebula which is generally visible to the human 

 eye with an ordinary telescope. (Fig. 2.) If we were dealing with 

 incandescent gas, the incandescent gas ought to leave off sud- 

 denly ; but all round this nebula, where th.re appears to be 

 nothing at all, the longer exposure brings before us other por- 

 tions of the nebula just as rich in details, just as exquisite in 

 their variety and tone as those ordin.irily seen with the naked eye. 

 Such a condition as that cannot be brought about by a mere 

 homogeneous mass of gas at high temperature, but we can explain 

 it quite easily by assuming that in such .i nebula as that we are 

 dealing with, the luminosity is brought about by disturbances, 

 these disturbances giving rise to collisions among the particles 

 which are apt to collide and give out luminosity. The nearer 

 they are to the centre of gravity of the swarm, the greater will 

 be their chance of collision, and the greater will he the lumin- 

 osity of their central portion. 



.Still another consideration. Astronomers, since the time 

 of Rutherfurd, who was the first to begin stellar spectroscopic 

 work in the United States, between i860 and 1S70, have 

 established many different classes of stars as defined by the 

 chemical substances of which their atmospheres seem to be 

 composed, so far as spectioscopic observations enable us to 

 determine their composition. One gioup of stars is remarkable 

 for the presence of hydrogen in enormous quantities: we as- 

 sume that because the lines of hydrogen are inordinately thick. 

 In another we get not so much hydrogen, although it is still there, 

 but the predominant substance is iron. In other stars we get 

 little hydrogen, if any, apparently no iron, but carbon in 

 enormous quantities, and again there is another substance, the 

 quantity of which varies enormously, and that is calcium. Now, 

 if stars contain all these diflerent substances, and if they 

 represent epochs of evolution, they must be produced from 

 something which actually or potentially contained these sub- 

 stances, so that there again you get a 

 considerable argument iti favour of the 

 chemical complexity of the nebula;. 



Finally, we reach the second point. It is 

 now generally conceded that the first stage 

 in the development of cosmicai bodies is 

 not a hot gas, but a swarm of cold meteor- 

 ites. From the point of view of evolution, 

 keeping well in touch with the laws of 

 iliermodynamics, the nebula; must begin 

 cool if they are to develop into hot stars. 

 J. NOK.MAN LOCKYER. 

 {To U continuta). 



A 



Fig. 31. — Orion nebula photographed with short and long expciarei 



the nebula which we see ordinarily in our telescopes is only a 

 very, very small fraction of the real nebula as it really exists, 

 when we can get at it under the best possible observing condi- 

 tions. .Many of you, I hope, have seen the nebula of Orion in 

 an ordinary telescope. Mere it is as it has been photographed 

 by means of a telescope powerful enough to give us the brighter 

 portions. Here is arolher photograph ol the nebula exactly on 



NO. 1329. VOL. 51] 



NEW COMPOUNDS OF PHOS- 

 PHORUS, NITROGEN, AND 

 CHLORINE. 

 SERIES of new compounds of phos- 

 phorus, nitrogen, and chlorine, and 

 likewise a series of acids derived from 

 ihoni, have been discovered by Mr. H. N. 

 Slokes, and an account of them is con- 

 tributed to the American Chemical Jour- 

 iir.l. .\ famili.ir compound of the three 

 elements in question, the chlorophos- 

 phuret of nitrogen, discovered by Liebig 

 n 1832, has been the subject of frequent 

 study, and its nature has comparatively 

 riccntly been very fully demonstrated by 

 Dr. Gladstone. It has been shown, from 

 vapour density determinations, that this 

 remarkably stable compound, which may 

 be distilled in steam and boiled with 

 acids and alkalies without appreciable 

 change, possesses the molecular composition PiNjCl,,. Mr. Stokes 

 now shows that this substance is only one of a homologous series 

 of compounds having the general formula (PNCK,)n, and that 

 these are the chlorides of a series of acids (PNO.JIJn, which 

 he terms mctai)hosi>himic acids. The second member of the 

 series, (PNCIj)4, has been isolated from the product of Dr 

 Gladstone's reaction for the preparation of (PNCUlg, that be 



