27 V.-Laboratory Memorandu. By ROBERTWARINGTON, Junr. Assistant to Professor of Chemistry R. A. College Cirencester. I. The Action of Ferricyanide of Potassium upon Ferric Salts.-Most works on chemistry describe this reaction as producing a “dark-brown,” or reddish-brown” fluid and state that no preci- pitate is formed;* some BIitnuals however tell usthat a “greenish,” or even a “dark-green” liquid is the resu1t.t These statements are all incomplete and may possibly mislead. First as to the colour. The dark-brown liquid is produced only when the ferricyanide of potassium is in excess of the ferric salt ; the tint in this case is a dark sherry-brown. When the propor- tions are reversed and a drop or two only of the ferricyanide mixed with an excess of ferric chloride a brilliant emerald-green tint of a slightly bluish tinge is the result.If the proportion of ferri- cyanide of potassium is then gradually increased the colour is first changed to sap-green then olive-brown and at last reaches the dark sherry tint before described. The whole series of colours is well seen if a single drop of ferricyanide of potassium is allowed to run down the side of a test-tube half filled with dilute ferric chloride ;a brown ring forms at the surface where the ferricyanide is in excess from which streaks of colour slowly descend their tint changing from brown to bright green as they fall further towards the bottom. It appears then that the colour produced on mixing an alkaline ferricyanide and a ferric salt is essentially dependent on the pro- portion of each which may be present.Lastly as to the statement that the liquid remains clear. This is true only when the solution is sufficiently acid ;basic ferric salts afford an immediate pale brown precipitate with ferricyanide of potassium . These results were all obtained with a freshly prepared solution of the ferricyanide. 11. The Solubility of Magnesia in 9lkaline Salts.-That magnesia dissolves in salts of ammonium with evolution of free ammonia is a sufficiently familiar fact. That the presence of ammonium- salts hinders or entirely prevents the precipitation of hydrated magnesia by an alkali is equally well known; but it is not per-* So Rose Fresenius and Gmelin. t ThusMiller and Northcote and Church.WARINGTON S LABORATORY MEMORANDA. haps so generally understood that salts of potassium and sodium have an exactly similar effect though in a far smaller degree The action of ammonium-salts in preventing the precipitation of magnesia is usually explained as owing to the formation of double salts of magnesium and ammonium. If this theory be correct then the presence of uny soluble base forming double compounds with magnesium ought to have a similar effect and consequently we should expect that salts of potassium and sodium would in this particular behave like salts of ammonium. Magnesia dissolves to a small extent in a strong solution of a salt of potassium and also in salts of sodium the alkaline salt being in this case evidently decomposed as the liquid strongly reddens turmeric paper.The following experiments were made as to the faculty of alkaline salts to prevent the precipitation of hydrated magnesia. Two equal volumes of a solution of chloride of magnesium were taken; in one of these 200 grs. of chloride of potassium were dis- solved; both were then treated with equal volumes of a solution of caustic potash; and after standing some hours in a warm place filtered and the magnesium in the filtrate (that there- fore which the potash had failed to precipitate) estimated by phosphate of sodium in the usual way. The experiment with chloride of potassium yielded 1.54 grs. oE Mg,P,O, the other only 1.39 grs.; the potassium-salt had clearly then to a slight extent prevented the precipitation of magiiesia.An experiment with chloride of sodium yielded a similar result. Trial was also made with sulphate of magnesium the sulphates of potassium and sodium being added. The amounts of magnesium unprecipitated by the potash are shewn by the following weights of pprophos-phate obtained :-With sulphate of magnesium alone . . .. . . *15grs. and sulphate of potassium *5O ,, JJ J? >s >) and sulphate of sodium. . -56 , Another experiment in which caustic soda was the precipitant gave similar results. With sulphate of magnesium alone . . .. . . -94 grs. 9 11 and sulphate of potassium 1.15 , J J> and sulphate of sodium. 1.35 , When these experiments were repeated with an increased quantity of caustic potash or soda the effect here noted practically WARINBTON’S LhBORllTORY MEMORANDA.disappeared a trace oiily of magnesium being found in any filtrate ; this indeed took place when the amount of potash was increased by one-half or even less. The excess of potash doubtless decom- posed the double salt first formed in the same way that a large excess of ammonia precipitates hydrated magnesia even in the presence of sal-ammoniac. It appears then that salts of potassium and sodium do really hinder the precipitation of hydrate of magnesium,* though to not nearly tlie same extent as salts of ammonium ; a result which well accords with theory as the double compounds of magnesium with tlie former salts are well known to be far less stable than those with the latter.Another fact in connection with the soliibility of magnesia in alkaline salts is the decomposition of this solittion by water under certain coiiditions For instance if a strong solution of magne-sium be mixed with sal-ammoniac and free ammonia the pro. portion of the two latter being so adjusted as just to prevent pre cipitation on standing the equilibrium of the mixture thus formet! is destroyed by the addition of water and maguesia is precipitated In one experiment a mixed solution of the chlorides of magnesinni and ammoiiium was treated with twice its volume of the strongest ammonia; at the end of twelv; hours the solution was still per- fectly clear An exactly similar mixture was prepared and diluted with water to three times its bulk; at the end of twelve hours 8 considerable precipitate of magnesia had fallen.Salts of ammonium appear then to hinder the precipitation of magnesia in proportion to their conceutration. This fact serves to explain a phenomenon sometimes observed when making determinations of phosphoric acid. If the washings of the phosphate of magnesium and ammonium are allowed to mix with the main filtrate an opaque ring is often formed at the junction of the two liquids alarming the operator with the idea that his precipitate is washing through ; the washings however if collected separately are found to be perfectly clear. The cause of this appearance is probably the precipitation of the excess of magnesia on diffusion into the wash-water. * According to L o n g c h a m p (Ann. Chin Phys. [3] xii. 255.) the precipitation of carbonate of magnesium is less complete in proportion to the amount of alkaline salts present.