DETECTION OF DYES. 663

red liquid, and leaves a blue bottom. Nitric acid extracts a similar liquid,but leaves a yellowish-brown ground.

5. Aniline Black.—Strong hydrochloric acid gives no immediate change,but after a quarter of an hour the cloth will be found to have taken a greenish-yellow, while the liquid is an olive-brown.

6. Lamp-Black.—Cannot be discharged. Ash white.

B. BLUES.

The principal blue colours of which we shall speak are :—

1. Logwood blue.

2. Prussian blue.

3. Aniline blue.

4. Indigo blue.

5. Mixed blues.

6. Ultramarine blue.

I. We begin by placing a piece of the cloth to be tested in a solution ofcitric acid, or in hydrochloric acid treated with water.

a. The colour changes to red or orange. Logwood.

b. The colour does not change. (In this case any of the other three

dye-wmres may be present.)

II. Another portion of the cloth is placed in a solution of chloride of lime.

a. Colour unaltered .Prussian blue.

b. Sample discoloured or turned yellowish .Blues 3 or 4.

III. In order to distinguish between these two blues we put a third swatch

into a solution of caustic soda.

a. Sample discharged or altered .Aniline.

b. Sample unchanged .Indigo.

We may satisfy ourselves of the correctness of our results by the applicationof the following corroborative tests: —

The Blue of Logwood is reddened by acids and restored by caustic soda.The cloth when burnt leaves white or grey ash ; the whiteness being due tothe alumina of the alum used as a mordant, and the grey tint caused by theoxide of copper contained in the blue vitriol used with the wood. If copperis present the edge of the flame appears green during combustion.

Prussian Blue being a cyanide of iron leaves on combustion a red ash,oxide of iron, more abundant in proportion as the blue is deeper.

Indigo Blues contain no mineral matter, and leave no ash except that ofthe cloth. They may be either vat-colours, China blues, pencil blues, orextradt of indigo. These are distinguished as follows:—

1. Indigo, Vat.—A piece of clean white silk rubbed on the cloth becomessmeared. Sulphuric and muriatic acids, and carbonates of soda and potash,produce no change. Strong nitric acid destroys the blue, leaving a clearyellow, if no other colouring matter is present. Chlorine destroys the colour.

2. Indigo, China Blue.—Behaves like a vat-blue. If the colour isdestroyed with chlorine, the cloth is not dyed on dipping in logwood liquor.

3. Indigo, Pencil Blue.—Behaves like the two former, but if dipped inlogwood liquor after the colour has been destroyed with chlorine it is dyed red.

4. Extradt of Indigo.—Unlike a vat-blue, it does not soil white silk onfridtion. Strong alkalies dissolve the colour, forming a blue solution. Nitricacid destroys it more rapidly than a vat-blue.