latter portion, divided by 2, gives the joint per centage amount of carbonated and pure alkali (expressed as carbonate). The number of centigrammes of the carbonic acid evolved from the first portion, divided by 2, gives the per centage amount of the carbonated alkali. The difference between the two figures indicates accordingly the quantity of carbonated alkali which was originally present in the examined potash or soda, in the form of caustic alkali. The corresponding quantities of potass or soda are found most readily by multiplying this difference with 0.6817 in the case of potass, and with 0.5886 in the case of soda. II. CHLORIMETRY. § 137. The chloride of lime, or bleaching powder of commerce, is a mixture of hydrate of lime with hypochlorite of lime and chloride of calcium; the normal chloride of lime contains the two latter constituents in equal proportions. The action of an acid upon chloride of lime effects the liberation of the whole of the chlorine present. 3 3 Ca O, CI O+ Ca Cl+2 S O ̧=2 Ca O, S O ̧+2 Cl. The bleaching powder of commerce varies exceedingly in the proportional amount of chlorine which it yields when treated with acids. Now, as the commercial value of this article, which is manufactured on an immense scale, depends entirely upon the proportional amount of chlorine which it contains, it was very natural that chemists should endeavour to devise some simple method to determine the available amount of chlorine present in any given sample of bleaching powder. The various methods proposed with the view of effecting their object have collectively received the name of " Chlorimetry.” I will here describe three different methods which yield equally satisfactory results. These methods are valuable not simply in a commercial, but also in a purely scientific point of view, since they will serve to determine with great accuracy the amount of free chlorine present in a given fluid. A. METHOD OF DETERMINING THE AMOUNT OF AVAILABLE 1. This method is based upon the circumstance that one equivalent of chlorine converts one equivalent of protochloride of mercury into perchloride. Hg, Cl+Cl 2 Hg Cl. 2. It follows accordingly that the amount of protochloride of mercury converted into perchloride indicates at once the amount of chlorine which has served to effect this conversion. If we take, therefore, a fluid which holds a known quantity of protochloride of mercury in suspension, and add to it solution of chlorine until the whole of the protochloride of mercury is dissolved as perchloride, we know at once the proportion of chlorine contained in the solution which has served to effect this conversion. 3. The most simple and accurate method of preparing a fluid holding in suspension a definite and known amount of protochloride of mercury, is based upon the following principle. a. 2945 parts of protochloride of mercury require 443 parts of chlorine to be converted into perchloride. b. 734 parts of chloride of sodium are required to form 2945 parts of protochloride of mercury. c. 734 parts of chloride of sodium (the chlorine being transferred to protoxide of mercury) correspond accordingly to 443 parts of chlorine, or, what comes to the same point, 165.7 parts of chloride of sodium correspond to 100 parts of chlorine. d. Now if 1.657 grammes of chloride of sodium be weighed off and dissolved, and solution of protonitrate of mercury be carefully added until the formation of a precipitate ceases, the quantity of the solution of protoxide of mercury will exactly correspond to 1.657 gramme of common salt, or 1.00 gramme of chlorine; if another quantity of the solution of protonitrate of mercury, of exactly the same volume as the amount used in the process, be now mixed with water until the whole volume corresponds to 100 volumes of any accurately graduated glass measure, a fluid will be produced of which every volume contains exactly as much protoxide, or, if the solution be previously precipitated with common salt, protochloride of mercury, as will be converted respectively into peroxide and perchloride, by 0.01 gramme of chlorine. 4. Preparation of a solution of protoxide of mercury of known and definite amount. According to the principle developed in 3, a solution of this kind is best prepared as follows: a. 5 grammes of chemically pure ignited chloride of sodium are weighed off, and dissolved in water; the solution is poured into a measure-glass, and as much water added as will make the volume of the whole fluid correspond to 400 volumes of the graduated dropping-tube, illustrated by Plate XLIV. b. 50 volumes (or less or more according to the degree of concentration of the fluid) of a solution of protonitrate of mercury, are measured off by means of a graduated syphon, of which the scale corresponds exactly with that of the dropping-tube; this measured portion of the solution is poured into an eight-ounce bottle, provided with a glass stopper, and is then finally diluted with water until the weight of the whole liquid is about five ounces. c. The graduated dropping-glass is now filled with the solu- tion of chloride of sodium of a., and the latter is then cautiously added to the solution of the protonitrate of mercury (b.) until the last drop added ceases to produce a precipitate. The bottle is to be placed in hot water during this operation, and agitated after every fresh addition of solution of chloride of sodium, in order to promote the subsidence of the precipitate. Should the operator happen to add an excess of chloride of sodium, which will sometimes occur, he need simply add a few more volumes of the solution of protonitrate of mercury, and be more careful in the renewed addition of the chloride of sodium solution. d. The measures of the chloride of sodium solution which have been used in this process, are accurately read off, and the amount of solution of mercury which corresponds to 1.657 gramme of common salt, is calculated from the resulting figure. Thus, for instance, let us suppose that 80 volumes of the solution of chloride of sodium have been required for the precipitation of 50 volumes of the solution of protonitrate of mercury: the calculation would proceed as follows: a. 400 volumes of solution of Na Cl: 5 grammes of Na Cl :: 80 volumes: x-x=1 gramme; B. 1 gramme of Na Cl: 50 volumes of solution of protonitrate of mercury: 1.657 of common salt: x-x-82.85; which means that 82.85 volumes of the solution of protonitrate of mercury correspond to 1.657 gramme of chloride of sodium or to 1.00 gramme of chlorine. We have accordingly to add 17.15 volumes of water to every 82.85 volumes of the solution of protonitrate of mercury 82.85 100.00 to produce a test fluid of the desired strength, i. e. of which every one volume corresponds to 0.01 gramme of chlorine. It is more advantageous still, to mix this fluid now with an equal volume of water, when every one volume will correspond simply to 0.005 gramme, and every 100 volumes to 0.5 gramme of chlorine. In the following instructions for the performance of the analysis, I shall assume the test solution to possess the latter degree of concentration. 5. Performance of the analytical process. a. 5 grammes of the sample of chloride of lime under examination are weighed off, finely triturated with a little water, and subsequently rinsed into a cylinder, which has been previously prepared for the purpose for which it is intended, by measuring into it 200 volumes of water from the graduated dropping-tube, and scratching a mark into the glass at the exact spot to which these 200 volumes of water reach. The cylinder is then filled with water up to the mark, and the mixture agitated. b. 100 volumes of the solution of protonitrate of mercury (corresponding to 0.5 gramme of chlorine) are measured off by means of a graduated syphon; they are then poured into a beakerglass, and diluted with water; an excess of solution of common salt is added, and finally a small portion of hydrochloric acid, which must be perfectly free from admixture of chlorine or sulphurous acid. c. The graduated dropping-tube is now filled up to the 0 point with the well-shaken solution of chloride of lime, (a.), which latter is then dropped cautiously and with constant stirring into the fluid of b. (which must invariably remain acid) until the mixture appears perfectly clear. The per centage amount of chlorine contained in the examined sample of bleaching powder may now be at once found, simply by dividing 2000 by the number of volumes of the solution of the latter which have been used in the process. The calculation proceeds as follows: The amount used of the chloride of lime solution corresponds to 0.5 grammes of chlorine; how much chlorine do the whole 200 volumes of this solution contain? Let us suppose we have used 90 volumes of the chloride of lime solution. CHLORINE BY MEANS OF ARSENIOUS ACID (after GAY-LUSSAC, slightly modified). 1. This method is based upon the circumstance that the con |