alkaline acetate (that is, a mixture of soda or ammonia with excess of acetic acid), and then a drop or two of solution of perchloride of iron; yellowish-white ferric phosphate (Fe,2PO) is precipitated.

Too much of the ferric chloride must not be added, or ferric acetate will be produced, in which ferric phosphate is to some extent soluble.

To remove the whole of the phosphoric radical from solution, add ferric chloride so long as a precipitate is produced, and then boil the mixture; ferric phosphate and ferric subacetate is precipitated.

To obtain confirmatory evidence of the presence of phosphate in this precipitate and to separate the phosphoric radical as a pure unmixed phosphate, collect the precipitate on a filter, wash, drop some solution of ammonia on it, then sulphide of ammonium, and finally wash with water; black ferrous sulphide remains on the filter, while phosphate of ammonium is found in the filtrate. To the filtrate add a mixture of solutions of sulphate of magnesium and chloride of ammonium, and well stir; ammoniomagnesian phosphate is precipitated.

The above reaction is useful in the analysis of bone-earth, other earthy phosphates, phosphate of iron, &c. which are insoluble in water. Only arseniates give similar appearances; but the acid solution of these may be decomposed by sulphuretted hydrogen (HS), especially after agitation with sulphurous acid and subsequent ebullition.

Other Analytical Reactions.-Solutions of barium and calcium salts give, with aqueous solutions of phosphates, white precipitates of the respective phosphates BaHPO,, or Ba ̧2PO, and CaHPO4, or Ca,2PO1, all of which are soluble in acetic and the stronger acids.

The foregoing acids contain the only acidulous radicals that are commonly met with in analysis or in ordinary pharmaceutical operations. There are, however, many others which occasionally present themselves, and of which the student should have some knowledge; the chief of these will now be shortly noticed. They are arranged in alphabetical order to facilitate reference.

BENZOIC ACID (HC,H,O,).—Slowly heat a fragment of benzoin in a test-tube; benzoic acid (Acidum Benzoicum, B. P.) rises in vapour and condenses in small white crystalline plates on the cool sides of the tube. Or boil the benzoin with one-fourth its weight of lime, filter, concentrate, decompose the benzoate of calcium by hydrochloric acid, collect the precipitated benzoic acid, press between paper, dry, and sublime in a tube or other vessel.

There is always associated with the product a minute quantity

of a volatile oil of agreeable odour.

To a little benzoic acid add a few drops of solution of ammonia; it readily dissolves, forming benzoate of ammonium (Ammonia Benzoas, B. P.) (AmC,H ̧02).

On evaporation, crystals of an acid benzoate (or, ammonia being added, a normal benzoate) of ammonium are deposited.

The following are the tests for benzoic acid :-To a portion of the above solution of benzoate of ammonium add a drop or two of sulphuric or hydrochloric acid; a white crystalline precipitate of benzoic acid separates. To another portion, carefully made

neutral, add a drop or two of neutral solution of perchloride of iron; reddish ferric benzoate separates. Benzoic acid is distinguished from an allied body, cinnamic acid (occurring in Balsam of Tolu &c.), by not yielding hydride of benzoyl (C,H,OH) (oil of bitter almonds) when distilled with chromic acid—that is, with a mixture of red chromate of potassium and sulphuric acid.

CYANIC ACID (HCyO). The valuable reducing-power of cyanide of potassium (KCy) (or ferrocyanide) (KFcy) on metallic oxides &c. is due to the avidity with which it forms cyanate (KCyO). Fuse a few grains of cyanide of potassium in a small porcelain crucible and add powdered oxide of lead; a globule of metallic lead is at once set free, excess of the oxide converting the whole of the cyanide of potassium into cyanate of potassium. Cyanate of potassium (KCNO) treated with sulphate of ammonium yields cyanate of ammonium (NH,CNO); and solution of cyanate of ammonium, when simply heated, changes to artificial urea (CHN2O), the most important constituent of urine and the chief form in which the nitrogen of food is eliminated from the animal system.

FORMIC ACID (HCHO,).-The red ant (Formica rufa) and several other insects, when irritated, eject a strongly acid, acrid liquid, having a composition expressed by the above formula, and which has appropriately received the name of formic acid; it is also contained in the leaves of the stinging-nettle. It may be artificially prepared by heating equal weights of oxalic acid and glycerin to a temperature of from 212° to 220° for fifteen hours. The glycerin has, apparently, no chemical action, but, for some unknown reason, induces decomposition of the oxalic acid at a lower temperature than would otherwise be necessary; at a higher temperature the formic acid itself is decomposed. On distilling the mixture with water the formic acid slowly passes over. The dilute acid may be concentrated by neutralizing with carbonate of lead, filtering, evaporating to a small bulk, collect

L

ing the deposited crystalline formiate of lead, drying, decomposing in a current of sulphuretted hydrogen, and separating the resulting syrupy acid, or distilling the formiate of lead with strong sulphuric acid.

It may be instructively but not economically prepared by the oxidation of methylic alcohol (wood-spirit), just as acetic acid and valerianic acid are obtained from ethylic alcohol and amylic alcohol respectively.

Formic acid does not char when heated alone or with sulphuric acid, but splits up into carbonic oxide gas and water. It is recognized by this property and by its reducing action on salts of gold, platinum, mercury, and silver. It is solid below 32°.

GALLIC ACID.-See Tannic Acid.

HYDROFERROCYANIC ACID (H Fe"Cye, or H,Fcy"""). -The ferrocyanide of most interest is that of potassium, the yellow prussiate of potash (Potassa Prussias Flava, B. P.) (K,Fey), the formation of which was alluded to in connexion with hydrocyanic acid (see page 181). It cannot be regarded as simply a double salt of cyanide of potassium with cyanide of iron (FeCy2, 4KCy), its chemical properties being entirely different from either of those substances; moreover, unlike cyanide of potassium, it is not poisonous. Most of its reactions point to the conclusion that its iron and cyanogen are intimately united to form a definite quadrivalent radical appropriately termed ferrocyanogen (FeCy, or Fcy).

Many of the ferrocyanides are insoluble, and are therefore precipitated when solution of ferrocyanide of potassium is added to the various salts. Those of iron and copper being of characteristic colour, are adopted as tests of the presence of the metals or

4

of the ferrocyanogen, as the case may be. To solution of ferrocyanide of potassium add a ferric salt; ferrocyanide of iron (Fe Fcy) (Prussian blue) is precipitated. To another portion add solution of a copper salt; reddish-brown ferrocyanide of copper (Cu,Fey) is precipitated. The ferrocyanogen in ferrocyanide of potassium is broken up when the salt is heated with sulphuric acid, carbonic oxide being evolved if the acid is strong, and hydrocyanic acid if weak :

HYDROFERRIDCYANIC ACID (H.Fe"",Cy12, or HIFdey1).-Pass chlorine gas through a few drops of solution of ferrocyanide of potassium until the liquid ceases to give a blue precipitate when a minute portion is taken out on the end of a glass rod and brought into contact with a drop of a dilute solution of a ferric salt; it now contains ferrideyanide of potassium (K.Fe"",Cy129 or K1Fdcy11), red prussiate of potash (B. P.), as it is termed, from the colour of its crystals:

2K' Fe"Cy's Cl2

6

2

=

2K'C1'K' Fe"",Cy'12"

The removal of two atoms of potassium from the ferrocyanide is the only change of composition that occurs; but the ferrocyanogen is altered in quality, its iron passing from the ferrous to the ferric condition, from bivalent to trivalent activity, a condition in which it no longer precipitates ferric salts, but, on the other hand, gives a dark-blue precipitate with ferrous salts. To a portion of the solution add solution of ferrous sulphate; a precipitate falls. This precipitate is ferridcyanide of iron (Turnbull's blue), Fe",Fe"",Cy'12, or Fell Fdcy.

K Fdey3FeSO1 = Fe,Fdcy + 3K,SO.