476 



NATURE 



[Septemher 17, 1896 



Berzelius himself; how lunj; the chli)iiiie conlioveisy tlividcci the 

 chemical world ; how triumphantly Davy emerged from it ; how 

 completely his views were recognised ; and how very instru- 

 mental they have been in advancing theoretical chemistry. 



The hope, hr)we\er, which Davy expressed in that same his- 

 toric paper, " that these new views would perhaps facilitate one 

 of the greatest problems in economical chemistry, the decompo- 

 sition of the muriates of soda and potash," was not to be realised 

 so soon. Although it had changed its name, chlorine was still 

 for many years manufactured by heating a mixture of salt, man- 

 ganese, and sulphuric acid in leaden stills, as before. 



This process leaves a residue consisting of sulphate of soda 

 and sulphate of manganese, and for some time attempts were 

 made to recover the sulphate of soda from these residues, and to 

 use it for the manufacture of carbonate of soda by the Le Blanc 

 (jrocess. On the other hand, the Le Blanc ]5rocess, which had 

 been discovered and put into practice almost simultaneously with 

 Berthollet's chlorine process, decomposed salt by sulphuric acid, 

 and sent the muriatic acid evolved into the atmosphere, causing 

 a great nuisance to the neighbourhood. 



Naturally, therefore, when Mr. William Gossage had suc- 

 ceeded in devising plant for condensing this muriatic acid, the 

 manufacturers of chlorine reverted to the original process of 

 Scheele, and heated manganese with the muriatic acid thus ob- 

 tained. Since then the manufacture of chlorine has become a 

 bye-product of the manufacture of soda by the Le Blanc process, 

 and remained so till very recently. 



For a great many years the muriatic acid was allowed to act 

 upon native ores of manganese in closed vessels of earthenware 

 or stone, to which heat could be applied, either externally or 

 internally. These native manganese ores, containing only a cer- 

 tain amount of peroxide, converted only a certain percentage of 

 the muriatic acid employed into free chlorine, the rest combining 

 with the manganese and iron contained in the ore, and forming 

 a brown and very acid solution, which it was a great difficulty 

 for the manufacturer to get rid of Consequently, many attempts 

 were made to regenerate ]")eroxide of manganese from these 

 waste liquors, so as to use it over again in the production of 

 chlorine. 



These, however, for a long time remained unsuccessful, 

 because the exact conditions for super-oxidising the protoxide 

 of manganese by itieans of atmospheric air were not yet 

 known. 



Meantime, viz. in 1845, Mr. Dunlop introduced into the 

 works created by his grandfather, Mr. Charles Tennant, at St. 

 Rollox, a new and very interesting method for producing 

 chlorine, which was in a certain measure a return to the process 

 used by the alchemists. 



Indeed, the first part of this process consisted in decomposing 

 a mixture of .salt and nitre with oil of vitriol — a reaction that 

 had been made use of for so many centuries ! The chlorine so 

 obtained is, however, not pure, but a mixture of chlorine with 

 oxides of nitrogen and hydrochloric acid, which Mr. Dunlop 

 had to find means to eliminate. 



For separating the nitrous oxides, Mr. Dunlop adopted the 

 method introduced twenty years before by the great Gay-Lussac 

 in connection with vitriol-making, viz. absorption by sulphuric 

 acid, and the nitro-sulphuric acid thus formed he also utilised 

 in the same way as that obtained from the towers which still 

 bear Gay-Lus.sac's illustrious name, viz. by using it in the vitriol 

 process in lieu of nitric acid. He then freed his chlorine gas 

 from hydrochloric acid by washing with water, and so obtained 

 it pure. This ]>rocess possessed two distinct advantages : (l) it 

 yielded a very much larger amount of chlorine from the same 

 amount of salt, and (2) the nitric acid, which was used for 

 oxidising the hydrogen in the hydrochloric acid, was not lost, 

 because the oxides of nitrogen to which it was reduced answered 

 the purpose for which the acid itself had previously been em- 

 |)lciyed. But this process was very limited in its application, as 

 il could only be worked to the extent to W'hich nitric acid was 

 used in vitriol-making. 



The process has been at work at St. Rollox for over fifty 

 years, and, as far as I know, is there still in operation ; but I 

 am not aware that it has ever been taken up elsewhere. 



Within the last few years, however, several serious attempts 

 have been made to give to this ])rocess a wider scope by regene- 

 rating nitric acid from the nitro-sulphuric acid and employing it 

 over and over again to convert hydrochloric acid into chlorine, 

 f^uite a number of patents have been taken out for this purpose, 

 all employing atmospheric- air for reconverting the nitrous oxides 



into nitric acid, and differing mainly in details of apparatus and 

 methods of work, and several of these have been put to practical 

 test on a fairly large scale in this neighbourhood, and also in 

 Gla.sgow, Middlesbrough, and elsewhere. As I do not want to 

 kee]) you here the whole afternoon, I have ti> draw the line some- 

 where as to what I shall include in this brief history of the 

 manufacture of chlorine, and have had to decide to restrict 

 my.self to those methods which have actually attained the rank ol 

 manufacturing processes on a large scale. As none of the pro- 

 cesses just referred to have attained that position, you will excuse 

 me for not entering into further details respecting them. 



Mr. Dunlo|)'s process only jjroduced a very small portion of 

 the chlorine manufactured at that time at St. Rollox, the re- 

 mainder being made, as before, from native manganese and 

 muriatic acid, leaving behind the very offensive waste liquors I 

 have mentioned before, which increased from year to year, and 

 became more and more difficult to get rid of The problem of 

 recovering from these liquors the manganese in the form of ])er- 

 oxide Mr. Dunlop succeeded in solving in 1855. 



He neutralised the free acid and precipitated the iron present 

 by treating these liquors with ground chalk in the cold and 

 settling out, and in later years, filter-pressing the precipitate, 

 which left him a solution of chloride of manganese, mixed only 

 with chloride of calcium. This was treated with a fresh quantity 

 of milk of chalk, but this time under pressure in closed vessels 

 ])rovided with agitators and heated by steam, under which con- 

 ditions all the manganese was precipitated as carbonate of man- 

 ganese. This ])recipitate was filtered off and well drained, and 

 was then passed on iron trays mounted on carriages through long 

 chambers, in which it was exposed to hot air at a temperature 

 of 300° C. , the process being practically made continuous, one 

 tray at the one end being taken out of these chambers, and a fresh 

 tray being put in at the other end. One pa.ssage through the.se 

 chambers sufficed to convert the carbonate of manganese into 

 peroxide, which was used in place of, and in the same way as, 

 the native manganese. 



The whole of the residual liquors made at the large works at 

 St. Rollox have been treated by this process with signal success 

 for a long number of years. For a short time the process was 

 discontinued in favour of the Weldon process (of which I have 

 to speak next) ; but after two years Dunlop's process was taken 

 up again, and, to the best of my knowledge, it is still in opera- 

 tion to this day. It has, however, just like Mr. Dunlop's first 

 chlorine process, never left the place of its birth (St. Rollox), 

 although it was for a period of over ten years w ithout a rival. 



In 1866, Mr. Walter Weldon patented a modification of a 

 process proposed by Mr. William Gossage, in 1837, for recover- 

 ing the manganese that had been used in the manufacture 

 of chlorine. Mr. Gossage had proposed to treat the residual 

 liquors of this manufacture by lime, and to oxidise the resulting 

 protoxide of manganese by bringing it into frequent and intimate 

 contact with atmospheric air. This process — and several 

 modifications thereof subsequently patented — had been tried in 

 various places without success. Mr. Weldon, however, did 

 succeed in obtaining a very satisfactory result, possibly — even 

 probably — because, not being a chemist, he did not add the 

 equivalent quantity of lime to his liquor to iirecipitate the 

 manganese, but used an excess. However, Mr. Weldon, if he 

 was not a chemist at that time, was a man of genius and of great 

 perseverance. He soon made himself a chemist, and having once 

 got a satisfactory result, he studied every small detail of the re- '' 

 action with the utmost tenacity until he had thoroughly 

 established how this satisfactory result could be obtained on the 

 largest scale with the greatest regularity and certainty. 



He even went further, and added considerably to our the- 

 oretical knowledge of the character of manganese peroxide and 

 similar peroxides by putting forward the view that these 

 compounds ]iossess the character of w'eak acids. He explained 

 in this way the necessity for the presence of an excess of lime 

 or other l)ase if the oxidation of the jirecipitated |irotoxide of 

 manganese liy means of atmospheric air was to proceed at a 

 sufficiently rapid rate. He ])ointed out that the jiroduct had to 

 be considered as a manganite of calcium, a view which has since 

 been thoroughly proved by the investigations of Goergen and 

 others ; and it is only fair to state that Weldon's process is not 

 only a process for recovering the peroxide of manganese originally 

 used, but that he introduced a new substance, viz. manganite of 

 calcium, to be continuously used over and over again in the 

 manufccture of chlorine. 



Mr. Welclon had the good fortune that his ideas were taken 



NO. 1403, VOL. 54] 



