The following experiments were undertaken, in accordance with Professor Roscoe's wish, for the purpose of testing the accuracy of Lewes' results. The first step taken was that of writing to Mr. Lewes, to invite him to send the exact description of the way in which he had operated in obtaining his preparations, and to this he cordially responded. Sulphuretted hydrogen and sulphur dioxide gases were simultaneously passed into 3 litres of distilled water contained in a Winchester quart bottle, for about 32 hours, the sulphur dioxide being kept slightly in excess. A normal caustic potash solution was prepared, and the Wackenroder solution obtained as above, without separating its suspended sulphur, was titrated with it. 10 c.c. of Wackenroder solution required 5 c.c. of normal potash solution for complete neutralisation, or 2·5 c.c. for half neutralisation. To 3000 c.c. of Wackenroder solution, 750 c.c. normal potash solution were added, slowly, and with continual stirring. After standing some time, the solution was filtered, the filtrate passing through perfectly clear. This clear filtrate smelt strongly of sulphur dioxide, and a portion treated with excess of potash failed to give any precipitate of sulphur, possibly because the sulphur at once passes into combination with the sulphurous acid existing in the liquid, and in presence of the alkali forms thiosulphate. The filtered solution was now evaporated on the water-bath, when sulphur was deposited and sulphur dioxide evolved, and on treating a portion with potash, an abundant precipitate of sulphur was obtained. After concentrating on the water-bath, the liquid was placed in a partial vacuum over sulphuric acid, when in about eight days a crop of perfectly clear and transparent crystals was obtained, which on analysis proved to be anhydrous potassium tetrathionate. On further standing, more tetrathionate crystals were deposited from the mother-liquor, and after some time a third crop was obtained. On examining this crop under the microscope, part of the crystals clearly resembled the previous crops, but amongst them were seen some crystals of quite different form. It was further found that if from this mixed crop a crystal, now recognised by the above analysis to be the tetrathionate, was carefully picked out, dried in a small piece of filter-paper, placed in a watch-glass, dissolved in a little water, and then treated with a drop of caustic soda solution, not the slightest change appeared. The same process repeated in another watch-glass with a crystal of the different form, resulted in the immediate appearance of a white precipitate of sulphur. Since all the crystals in this crop were perfectly transparent, and yielded perfectly clear and neutral solutions, there can be no question, simple though the method may be, that the former, being crystals of potassium tetrathionate, the latter must be those of a totally different and less stable substance, decomposed by alkali. This mixed crop of crystals was analysed, and gave the following numbers: After some time, two other crops of mixed crystals were obtained, and these still showed on analysis the relation K: S :: 2 : 46. The next crop was composed of larger crystals, and the strange crystals were so abundant in quantity as to be easily discernible, and indeed a quantity was carefully selected and analysed, with the following result:— The water of crystallisation was determined by combustion with lead monoxide, and the difficulty of excluding traces of moisture probably accounts for the difference between the found and calculated percentages of water. These crystals thus analysed were colourless and transparent, and also dissolved in water to a perfectly clear neutral solution. The following are the results of the analyses of all the crops obtained, ending with the analysis of the selected crystals of pentathionate: Mr. H. Baker kindly determined the crystalline form of both the tetra- and penta-thionate. Examined microscopically the crystals of potassium tetrathionate and pentathionate are both seen to belong to the rhombic system, but differ entirely from each other, since the tetrathionate is most markedly hemimorphous, whilst the pentathionate is holohedral. The general form of the tetrathionate is that of a thick but flat crystal, terminated at one end by a pair of slant faces meeting sharply in a point, at the other by a slant face at right angles to the length of the crystal. The depolarising directions are parallel and at right angles to the length, and in the polariscope one of the two optical axes is just visible. The principal forms are Po, P, and P (no measurements having been made, the signs and are only used for distinction). Two faces of Po give the crystals their tabular appearance, and P is the hemimorphous form. The plane of the optical axes is coP. The pentathionate forms short thick prisms, four or six-sided, with flat terminal faces and small subsidiary pyramidal or domal faces. Forms coP.OP.coPo. Po. The depolarising directions are parallel and at right angles to the prism-edges. Other crystals showed more numerous faces, but could not be made out under the microscope. It was observed, in the course of several attempted preparations of potassium pentathionate from half-neutralised Wackenroder solutions, that on evaporation even in a vacuum over sulphuric acid, sometimes the liquid persistently remained turbid, even after many filtrations, and the crystals obtained in such cases were contaminated with sulphur and quite unfit for analysis, exactly according with Spring's experience in his attempt to repeat Lewes' method of preparation. It is not easy to say in what respect the mode of conducting the preparation when pure specimens were obtained differed from that in which an impure and less stable product was the result. XLIX.-Note on Pentathionic Acid in connection with the foregoing Paper. By WATSON SMITH. In the preparation of the Wackenroder solution by Mr. Shaw, to serve as material for the experiments he has detailed in the foregoing paper, it seemed of interest once more to determine whether this solution, as stated by Spring (Annalen, 213 [3], 352), when prepared so that sulphurous acid shall throughout the course of saturation be maintained in slight excess, really does or does not possess bleaching power for indigo solutions, and so set at rest an already sufficiently vexed question. Mr. Shaw, after scrupulously adhering to the condition specified above, viz., a maintained excess of sulphurous acid during saturation, obtained a Wackenroder liquid not bleaching even very dilute indigo solutions. He thus confirms the results of Smith and Takamatsu on this head. Now the importance of a confirmation or otherwise of this reaction will be manifest when it is remembered that Spring (loc. cit.) bases upon it a theory for the formation of tetrathionic acid in the Wackenroder solution, according to which theory thiosulphuric acid ought simultaneously to be formed, and it is to the presence of this thiosulphuric that Spring ascribes the bleaching power referred to, which he states he has observed in the Wackenroder liquid. The equation Spring gives to illustrate the theory thus set up, and which must stand or fall by the success or failure of the reaction in question, is :- (1) SO2 + H2O + S = HS.SO,H. (2) SO, + {HS = H2SO2 + [ S.SO,H A circumstance well worthy of record was observed further, viz., that the Wackenroder solution as freshly prepared is not only precipitated by alkalis, but also by acids, sulphur in both cases being separated, whilst on the other hand, the half-neutralised solution is not precipitated by alkalis in excess, although a precipitation is formed by acids, sulphur and sulphur dioxide being set free. Moreover the liquid obtained after concentration and filtration of the half-neutralised solution, yields, on treatment with acid, no precipitate of sulphur whatever; but on treating with alkalis, yields a considerable precipitate. The following is the explanation of these apparently paradoxical results. As is well known, a freshly prepared Wackenroder solution continues for a long time to deposit sulphur if left to itself, and if the mixed gases in the preparation have been passed into a saline solution (containing, e.g., sodium, magnesium, or calcium chloride) far more sulphur is precipitated than would be the case if pure water had been used. Stingl and Morawski (J. pr. Chem. [2], 20, 76), who demonstrated this fact, state that the cause of this action is a physical one, and the hastened precipitation on addition of hydrochloric or sulphuric acids is doubtless due to a similar cause. It is clear indeed from Stingl and Morawski's experiments that pure water should be used in the preparation of the Wackenroder solution, if an acid of maximum stability is to be obtained. In explanation of the non-precipitation of the half-neutralised Wackenroder liquid by alkalis, and its precipitation by acids, it may be mentioned at the outset that this liquid smelt strongly of sulphurous acid, and that sulphur had abundantly separated out on half-neutralising. The Wackenroder liquid, consisting of a solution of pentathionic acid, on half-neutralising with the dilute alkali, yields1st. Alkaline pentathionates. 2nd. Alkaline tetrathionates, formed by subsequent decomposition of the pentathionate. 3rd. Alkaline thiosulphates and sulphites, formed by subsequent decomposition of the pentathionate. 4th. Free sulphurous acid and sulphur, the latter arising primarily from the disintegration of the pentathionic acid in forming tetrathionates, and both former and latter secondarily, from that of the thiosulphates in the slightly acid solution. Now in presence of excess of sulphurous acid, no precipitate of sulphur appears in a solution of a pentathionate on adding an alkali |