The nature of the colouring matter which is employed in the preparation of litmus-paper has already been described at [page 353].

In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric-paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as Oxide of Silver.

In using test-papers, observe the following precautions:—they should be kept in a dark place, and protected from the action of the air, or they soon become purple from Carbonic Acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of Liquor Potassæ or Ammoniæ, or a grain of Carbonate of Soda to four ounces, the blue colour is restored. As the quantities which are tested for in Photography are often infinitesimally small, it is essential that the litmus-paper should be in good condition; and test-papers prepared with porous paper will be found to show the colour better than those upon glazed or strongly-sized paper. The mode of employing the paper is as follows:—Place a small strip in the liquid to be examined: if it becomes at once bright red, a strong acid is present; but if it changes slowly to a wine-red tint, a weak acid, such as Acetic or Carbonic, is indicated. In the case of the Photographic Nitrate Bath faintly acidified with Acetic Acid, a purple colour only may be expected, and a decided red colour would suggest the presence of Nitric Acid. In the Hypo fixing and toning Bath which has acquired acidity, the litmus-paper will perhaps redden in about three or four minutes.

Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with Sulphuric Acid, one drop to half a pint; or by holding for an instant near the mouth of a bottle containing Glacial Acetic Acid. In examining a Nitrate Bath for alkalinity by means of the reddened litmus-paper, at least five or ten minutes should be allowed for the action, since the change of colour from red to blue takes place very slowly.

REMOVAL OF SILVER STAINS FROM THE HANDS, LINEN, ETC.

The black stains upon the hands caused by Nitrate of Silver, may readily be removed by moistening them and rubbing with a lump of Cyanide of Potassium. As this salt however is highly poisonous, many may prefer the following plan:—Wet the spot with a saturated solution of Iodide of Potassium, and afterwards with Nitric Acid (the strong Nitric Acid acts upon the skin and turns it yellow, it must therefore be diluted with two parts of water before use); then wash with solution of Hyposulphite of Soda.

Stains upon white linen may be easily removed by brushing them with a solution of Iodine in Iodide of Potassium, and afterwards washing with water and soaking in Hyposulphite of Soda, or Cyanide of Potassium, until the yellow Iodide of Silver is dissolved out; the Bichloride of Mercury (neutral solution) also answers well in many cases, changing the dark spot to white ([p. 151]).

A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN DILUTE SULPHURIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.)

Real AcidReal AcidReal Acid
Specificin 100Specificin 100Specificin 100
Gravity.parts of theGravity.parts of theGravity.parts of the
Liquid.Liquid.Liquid.
1·848581·541·811573·391·712065·23
1·847580·721·804372·571·699364·42
1·846079·901·796271·751·687063·60
1·843979·091·787070·941·675062·78
1·841078·281·777470·121·663061·97
1·837677·461·767369·311·652061·15
1·833676·651·757068·491·641560·34
1·829075·831·746567·681·632159·52
1·823375·021·736066·861·620458·71
1·817974·201·724566·051·609057·89

A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN THE LIQUID NITRIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.)