x 34

DYEING AND CALICO PRINTING.

about 4 lbs. of peroxide of manganese per cubic foot, is then ready to be usedin the stills. There it reads upon hydrochloric acid, liberating chlorine, withreproduction of exadly such a residual solution as was commenced with.With that solution the round of operations is begun again ; and so on, timeafter time, indefinitely.

The quantity of lime which has to be put into the oxidiser, before the filtratefrom a sample of its contents ceases to give a manganese reaction, varies veryconsiderably. Recently precipitated protoxide of manganese dissolves veryappreciably in neutral solution of chloride of calcium, its solution thereincomporting itself with reagents exactly like solutions of manganese salts. Itdissolves also in solution of oxychloride of calcium,—that is to say, in solutionof chloride of calcium containing dissolved lime ; its solution in oxychlorideof calcium not giving the ordinary manganese reactions. Hence, even if allportions of the lime added to the chloride of manganese in the oxidiser werecapable of aCting on chloride of manganese equally readily, manganese couldnot cease to be so in solution as to be detectable by ordinary reagents untilmore than an equivalent of lime had been added,—until enough had beenadded, that is to say, not only to decompose all the chloride of manganese,but also to form a certain quantity of oxychloride of calcium. It is never thecase, however, that all portions of the lime used are capable of aCting onthe chloride of manganese with equal readiness. The lime used alwayscontains a larger or smaller proportion of particles coarser than the rest, whichcoarser portions cannot of course aCt so rapidly as the finer portions ; and asthe decomposition of the chloride of manganese requires to be completed asquickly as possible, those portions of the lime which will not ad upon itinstantly are scarcely allowed time to aCt upon it at all. These coarserportions of the lime thus contribute very little to the decomposition of thechloride of manganese, though they afterwards dissolve completely in the hotsolution of chloride of calcium, and then play their full part in the readionswhich take place during the subsequent blowing. The proportion of limewhich does not ad on the chloride of manganese varies with the source of thelime, and with the manner in which it is prepared ; so that the quantity oflime which has to be added to a charge of chloride of manganese liquor in theoxidiser, before the filtrate from a sample of the resulting mixture ceases tobecome coloured on addition of solution of bleaching-powder, varies fromabout 1*15 to i*45 equivalents. The further quantity of lime which is addedafter that point has been reached is now usually enough to raise the totalquantity to about 1*5 to r6 equivalents, being from one-half to six-tenths inexcess’of the quantity which adually takes part in the decomposition of thechloride of manganese.

“The results,” continues Mr. Weldon, “which we have latterly begun toobtain with these proportions of lime are such as our previous experience hadnot prepared us to exped. Until quite recently, our experience seemed toshow that whatever proportions of lime we might use in the oxidiser, we couldnot obtain produds containing less than an equivalent of base or bases perequivalent of peroxide of manganese in them. Now, however, we are regu-larly obtaining produds containing only between o-g and 07 equivalent ofbase, and we have occasionally got down nearly to 0*5 equivalent of base.With resped to the conditions which determine the formation of produds