June 1, 1894.] 



KNOWLEDGE 



133 



That we fail to find such a parallel arrangement of 

 diverging streams on either side of the Milky Way seems 

 to indicate that the stream of the Milky Way bears a 

 closer analogy to an open star cluster than to a cluster of 

 the centrally condensed type. 



The nebula about ij Argus is a very fine specimen of a 

 nebulous cluster with a central condensation associated 

 with dark structures and radiating streams of stars. Fig. 2 

 shows several streams of stars diverging from the central 

 mass. (I have indicated the place of some of the most 

 remarkable of these streams by scratches on the block.) 

 The streams are in most cases accompanied by narrow 

 black channels in the general nebulosity, which run parallel 

 to and alongside of the star streams. The reader should 

 examine these on the large plate. The radiating character 

 of the great dark rifts, which entirely cut out the light 

 from some parts of the central region of the cluster, 

 is also well shown in this small scale picture of the 

 nebula. 



It is interesting to compare it with the larger photograph 

 of the nebula reproduced in our first plate, as well as 

 with the large scale photographs of this nebular cluster 

 reproduced in the March and April numbers of Knowledge 

 for last year (1893). Sir John Herschel was the first to 

 point out the lines of stars bordering the edges of the 

 great dark rifts in this nebular cluster. There can be no 

 doubt about the intimate connection between the dark 

 structures and lines of stars in this cluster. 



The trifid character of the great dark rift which blots 

 out the central regions of the i; Argus nebula may be 

 worth noting in connection with the somewhat similar 

 though fainter trifid rift in the Hercules cluster, which 

 was first observed by the Earl of Eosse, and described by 

 him as " three dark lanes meeting in a point." They are 

 well shown in several protographs of the Hercules cluster 

 which have been taken by the Brothers Henry, Dr< Isaac 

 Eoberts, and others. It may be worth noting, also, that 

 there are some dark patches and dark structures in the 

 central parts of the cluster shown in Fig. 1. They are 

 just visible in our plate, but are better seen in the silver 

 print from Mr. Russell's negative. 



Fig. 3 shows some curious closed curves of stars forming 



a mesh work, and a circle 

 of stars surrounding a com- 

 paratively dark area. In the 

 light of other phenomena, 

 it seems not improbable 

 that such closed curves of 

 stars, which are found in 

 many parts of the Milky 

 Way, may be due to star 

 clusters, the central parts 

 of which are obliterated 

 by dark structures envelop- 

 ing and cutting out the 

 light from the central parts 

 of the cluster. Fig. 3 also 

 shows several narrow dark 

 channels in the general 

 nebulosity. Similar dark 

 channels may be traced rmming over nearly all the brighter 

 regions shown in our second plate. There are also several 

 broader branching dark structures, one of which (im- 

 mediately to the left or following side of the cluster 

 shown in Fig. 1) evidently extends in front of the cluster, 

 for its branches interfere with some of the streams of 

 stars from the cluster, a fact which renders it probable 

 that this cluster lies at some depth within the star stream 

 of the Milky Way. 



Fig. 3. — Meshwork of Stars to 

 the South of cluster shown iu 

 Fig. 1. 



LIQUID CHLORINE. 



By A. G. BLox.\5t. 



THE critical temperature of chlorine gas," or the 

 temperature below which it must be cooled before 

 it can be liquefied by pressure is about 38° C, so 

 that this gas may be counted among those which 

 are easily converted into liquids. Faraday first 

 produced liquid chlorine, and described its colour as dark 

 greenish-yellow ; its specific gravity was afterwards deter- 

 mined to be 1'33, and its boiling-point 33-6° C. 



It is very probable, however, that Faraday little dreamed 

 of liquid chlorine as an article of commerce to be bought 

 and sold by the hundredweight. He would, we imagine, 

 have been rather hard put to it to suggest a material for 

 the condensing pump and reservoir ; though, iu respect to 

 his memory, be it said that a very few experiments could not 

 have failed to furnish him with a solution to the problem. 



The difficulties attending the production of chlorine, 

 and the impossibility of transmitting it in the gaseous 

 state, have combined to render chloride of lime in this 

 country, and chloride of soda (eau (h\jarel) iu France, the 

 only forms in which this indispensable bleaching agent can 

 be put upon the market. When it is remembered that 

 chloride of lime at its best does not contain more than 38 

 per cent, of " available chlorine," the advantage to be 

 obtained in the matter of freight by transporting pure 

 liquid chlorine instead of bleaching powder will be apparent. 



The actual manufacture of liquid chlorine is now being 

 undertaken by a firm of alkali-makers at Salindres, the pro- 

 cess being conducted as follows, according to M. Fribourg : — 



The condensing pump is provided with a piston of 

 sulphuric acid, as being the packing best fitted to with- 

 stand the attack of the chlorine. To render such a piston 

 eifective, a pump of peculiar construction is of course 

 essential. This takes the form of a U-shaped tube of 

 cast-iron, lined with lead. The limb of this tube which 

 is to receive the chlorine is partly filled with strong 

 sulphuric acid, whilst the other limb contains petroleum 

 oil, and is provided with an ordinary piston. The upstroke 

 of the piston raises the petroleum, lowers the sulphuric 

 acid, and allows the chlorine to flow into the vacuum thus 

 formed, through a side tube provided with a leaden (?) 

 valve ; the downstroke compresses the chlorine through 

 another pipe into the refrigerated receiver, re-entry into 

 the generator being prevented by the leaden valve. 



Such is the somewhat bald information connecting the 

 compressing machinery, with which we have at present to 

 remain satisfied. The receivers in which the liquid is 

 transported are wrought-iron or steel cylinders, the necks 

 of which are furnished with T-pieces carrying two bronze 

 valves, the one connected with a tube reaching to the 

 bottom of the receiver and destined to serve for with- 

 drawing the liquid chlorine as such, the other serving for 

 the withdrawal of gaseous chlorine. The cylinder weighs 

 some 225 lbs., and is capable of containing 1 cwt. of 

 liquid chlorine, or nearly 525 cubic feet of the gas. The 

 pressure in the cylinder is 10 atmospheres at 35' C, but 

 they are tested up to 100 atmospheres. The anhydrous 

 chlorine has no action on the iron, bronze, and lead fittings. 



It is proposed to sell small cylinders containing 10 lbs. 

 of liquid chlorine for experimental use, so that the chemist 

 may look forward to a day in the near future when his gas- 

 generating flask shall be shelved once and for all, and his 

 oxygen, hydrogen, nitrogen, carbonic acid, sulphurous 

 acid, ammonia, chlorine, and sulphuretted hydrogen be at 

 hand, each in its steel or glass cylinder. 



* The article by M. Feiboubg on Liquid Chlorine, from which the 

 facts iu this article are taken, appeared in a June number of 

 Bulletin Soc. Chim. 



