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protocyanide of iron présent, and, in the second, you have no percyanide.

Simple as the production of Prussian blue appears, it requires much practice to préparé it with certaintyas a pigment where a given shade of blue is required. It is seldom found pure in commerce, being generally mixed with starch, chalk, or gypsum. Sometimes they are added with a view to déception, sometimes. in order to produce a lighter shade of colour.. The testing of a Prussian blue by Chemical means is not, therefore, a true criterion of its quality. The method usually employed consista in grinding in oil equalweights of a Prussian blue of known value and of the one to be tested, white lead is added to each, and the intensity of the colours compared.

The best quality of Prussian blue is obtained by mix-ing a dilute solution of red prussiate of potash with proto-salt of iron ; the second quality is made by mixing yellow prussiate of potash with pernitrate of iron. and is called Turnbulïs blue. Cheaper qualities are made by mixing solutions of yellow prussiate and proto-sulphate of iron (green copperas), which produces a pale-blue precipitate. This is transformed into Prussian blue by the addition of bleaching powder, which oxidises part of the iron, and transforme the protocyanide into percyanide. For still lower qualities, alum is mixed with the iron solution previously to the prussiate being added. Alumina is by this means mixed with the blue.

Prussian blue was first obtained on silk fabrics in 1811, by a professer of chemistry at Lyons, named Raymond, in conséquence of the high premium offered by the First Napoléon for the production ofa fast blue, as indigo could not at that time be imported into France. His process, with slight modifications, is followed at the présent day, although the colour is not much used,owing to the introduction of the aniline blues. It consists indip-ping the silk, for several hours, in a sait of peroxide of iron, when the oxide of iron becomes fixed in the silk, which is washed and dipped in a slightly acid solution of yellow prussiate of potash. Prussian blue is thus produced on the silk, which only requires washing to be ready for market. The only improvement made in this class of dying has been the addition of a persalt of tin to the iron-sait.

The production of Prussian blue on cotton or woollen fibres is effected. by a curious Chemical reaction. At a température of 212° Fahr., all acids, even the organic, such as oxalic, citric, and tartaric, as well as the acid sulphates, possess the property of decomposing the two prussiates. The potassium of the cyanide combines with oxygen of the water, and with the organic acid. The cyanogen thus liberated unités with the hydrogen of the water, forming prussic acid. The cyanide of iron liberated unités with the fibre of the cloth, and on the latter being passed into a weak solution of bichromate of potash or bleaching powder, or if it is left exposed to the air, part of the proto-cyanide of iron is converted into sesquicyanide, and Prussian blue is produced. As salts of tin greatly facili-tate the fixing of the prussiate on the cloth, chloride of tin is now generally mixed with the prussiate of potash. in this case prussiate oftin is produced. To this mixture > tartaric or oxalic acid is added ; it is then properly thickened and printed on the calico. When the design is dry, the fabric is submitted to the action of steam, and the blue is produced on the fabric. It then only requires to be passed through a bath of bichromate of potash, to develop the full depth of the shade.

' Ultramarine Blue is a most valuable pigment, both on account of its cheapness, and the brilliancy ofits colour. It is used extensively in many branches of trade—by the f calico printer in pigment printing, by the paper stainer f and manufacturer, the typographie and lithographie , printer, by the match manufacturer, the sugar refîner, t and by the house decorator. The value of the pigment t dépends on the fineness of the powder, and the ) brilliancy of its hue.

browns, maroons, and other dark shades, in conjunction I with other dye-stuffs. Its chief use is to top cheap i indigo blues on woollen goods ; this is effected by lightly dying the fabric with indigo (an expensive dye) and then i passing it through a bath of orchil, which gives to the I cloth a rich purple hue, similar in appearance to one 1 dyed wholly with indigo. I

Cudbear is a spécial préparation of orchil, first manu- t factured by Dr. Cuthbert Gordon, from whom it dérivés i its name. t

Litmus is obtained from the same lichens as those t employed for producing orchil, only lime and carbonate i of potash are added to the ground weed and urine ; in ( fact the process is very similar to that used in former t times to produce orchil. After three or four weeks, a t blue colour is fully developed, when it is mixed with sulphate of lime or chalk, dried, and is ready for market, i The chief employment of litmus is to communicate a ] peculiar tint to the cheese made in Holland. ;

Prussian Plue.—I shall now have the pleasure of draw- i ing your attention to one of the finest, brightest, and most permanent colours known. It was discovered i accidentally in 1710, by a colour manufacturer named : Diesbach, of Berlin, from which it derived its name. The process by which it was produced was kept a comparative secret until 1724, when Dr. Woodward showed how the colour had become a very profitable affair, and described a process by which it could be obtained. The process has in time undergone many improvements ; but I shall here confine myself to a description of it as now carried out. I shall be obliged here to enter slightly into theoretical chemistry.

Prussian blue is a combination of iron with cyanogen, a compound of carbon and nitrogen. There are two well-defined cyanides of iron corresponding to the two oxides, the proto-cyanide, Fe. Cy. and the sesquicyanide, Fez. Cy3. These two cyanides, combining together in different proportions, give rise to the various shades of Prussian blue found in commerce. But, strange to say, you cannot produce either of these compounds or Prussian blue by the direct combination of cyanogen with iron. Cyanogen must be first united with potassium, giving rise to cyanide of potassium, under the influence of which sait the iron combines with cyanogen, and the cyanide of iron thus formed in its turn combines with the cyanide of potassium, forming a double cyanide. There are two double cyanides of potassium and iron ; the first, called ferro-cyanide of potassium, or yellow prussiate of potash ; the second, ferri-cyanide, or red prussiate of potash. The formula may be represented as follows :—

Yellow prussiate .. Fe. Cy. - K Cy. Red Prussiate .... Fe.2 Cy.3 + 3KCy.

To produce Prussian blue from these salts, it is neces-sary to replace the potassium by iron. This is effected by adding a persalt of iron to the yellow prussiate or to the red prussiate, a proto-salt of iron. The iron of the iron sait replaces the potassium of the double salts, and double cyanides of iron are produced, as seen by the fol-lowing formulæ :—

Sesquichloride of

Yellow prussiate. iron. Prussian blue.

3 (Fe. Cy. K. Cy.) + Fe.a C1.3 = 3 Fe. Cy., Fe.2 Cy.3 Chloride of potassium.

+ 3 K. Cl.

Protochloride of

Red prussiate. iron. Prussian blue.

Fe.2 Cy. 3 K. Cy. + 3 Fe. Cl. = Fe.2 Cy.3, 3 Fc. Cy.

Chloride of Potassium.

+ 3 K. Cl.

That this is the case can be easily demonstrated, for if you mix together a solution of persalt of iron and one of red prussiate, which contains only percyanide. of iron, do Prussian blue is formed. Again, if you mix yellow prussiate of potash with protochloride of iron, you get do Prussian blue, because, in the first case, you have no

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