tributed throughout nature, although it occurs in small quantities. It is found in most rocks and soils; from thence it passes into the various vegetable organisms, these in their turn supplying it to animals, to whom it is strictly essential for the formation of their skeleton.

The bones of animals, which contain, after calcination and removal of the organic constituents, 77 per cent. of phosphate of calcium, constitute the great source whence phosphorus is obtained. They are digested with sulphuric acid (H, SO), which removes the greater part of their calcium as sulphate of calcium (Ca, SO), leaving an acid phosphate of calcium (CaH, PO1). This, when thoroughly dried and then ignited, is intimately mixed with one-fourth its weight of wood-charcoal, the mixture being afterwards placed in an earthen retort and strongly heated. Part of the oxygen of the phosphate of calcium is removed by the carbon, and phosphorus distils over. The annexed equation expresses this decomposition symbolically:

4(CaPO,)+5C=5CO+Ca, P2O,+2P.

Air is of course carefully excluded in this experiment.

The student should examine the following properties of phosphorus :

(a) Its "phosphorescence," the lambent greenish-blue light which it exhibits when exposed to the air in a dark place.

(3) Its ready fusibility: for this purpose a small piece should be placed in a flask, then be covered with water, and afterwards heated gently.

(7) Its ready inflammability when heated in the air.

(8) Its inflammability even under water, if melted and supplied with sufficient oxygen. For this experiment a few small pieces of phosphorus are to be placed at the bottom of a beaker in contact with some crystals of chlorate of potassium (KCl 0 ̧) (a salt which was mentioned when treating of oxygen). The fragments are then to be covered an inch or two deep with water, and afterwards sulphuric acid (H, SO,) poured upon them by means of a funnel-tube, so as to reach the chlorate before dilution with the water. The chloric acid thus set free (HCl 0,)

being a body which parts readily with its oxygen, is at once decomposed by the phosphorus, which burns vividly with formation of phosphoric acid.

Phosphorus, as it usually occurs, is a solid having the density of 1.83; it melts at 44° C., and boils at 290°, forming a vapour the density of which has been found to be 4.355. Phosphorus is insoluble in water, but excessively soluble in bisulphide of carbon (CS). Its affinity for oxygen is so very great, that if exposed to this gas it gradually combines with it, becoming converted into oxide; and if the temperature be but slightly raised, this affinity increases so greatly that the phosphorus enters into combustion. This element presents the peculiarity which has been stated to belong to sulphur, carbon, boron, and silicon, namely, that of allotropy, and which oxygen also probably presents, since ozone seems to be only a modification of that element. If the ordinary phosphorus is exposed to a heat of 215°-250° C. out of contact with the air, it becomes converted into the second variety (3-P), which, by further heating to the temperature of 260° C., may be reconverted into the original form. The differences between these two forms of the element are these:-ordinary phosphorus (a-P) is a wax-like solid, translucent and nearly colourless; its specific gravity is 1.83, and it combines with oxygen at 75° C. with inflammation, and is phosphorescent at ordinary temperatures; while the second variety (B-P) is a red uncrystallizable solid with an almost metallic lustre on the fractured surface, with a specific gravity of 2.089, and admitting of exposure to atmospheric air without combining with its oxygen, unless heated to 260°, the point at which the ordinary modification is regenerated; it is not in the least degree phosphorescent. A third variety of phosphorus (y-P) has been obtained by heating ordinary phosphorus considerably higher than its melting-point, and then suddenly cooling it. In this state it is black, but will recover its former translucence, &c. when fused and slowly cooled.

III. ARSENIC=As. The non-metallic element, or salt-radical arsenic (for so we may venture to call it, although since it has some claims to be considered a basic element we have already, in Chap.

II., mentioned it among those bodies), is a solid at the ordinary temperature.

Arsenic is sometimes found in a pure state in nature, but more frequently associated with metals, such as iron (Fe), copper (Cu), nickel (Ni), cobalt (Co), &c.; it is obtained in the form of an oxide by heating these compounds in a current of air, and from this the arsenic is separated by the action of carbon.

The student should observe—

(a) The volatility of this substance, and its peculiar physical characteristics, and particularly its garlic odour.

(6) Its ready conversion into a white crystalline sublimate (As, O,), when heated in a current of air.

Arsenic has a metallic appearance, with a steel-grey lustre. It crystallizes in tetrahedra, and is very brittle. Its density is 5.75 in the solid form, 10.39 in the gaseous. It volatilizes at 300° C. It is tasteless, insoluble in water, but soluble in hydrochloric acid (HCl). When powdered arsenic is thrown into a jar of chlorine gas, it combines with that gas, forming chloride of arsenic (As Cl1). Arsenic combines very readily with hydrogen to form arseniuretted hydrogen (As H,), but does not seem capable, like nitrogen, of forming a stable compound containing another equivalent of hydrogen, together with one equivalent of some such acid radical as chlorine, in which compound the group *YH, plays the part of a powerful basic radical. Some arsenic compounds have indeed been obtained formed on this type, but they are in general very unstable.

IV. ANTIMONY=Sb. This substance, which has already been noticed among the basic elements, may also be regarded as a body possessed of acid characters. In its combinations it closely resembles arsenic, and forms with hydrogen an analogous compound, antimoniuretted hydrogen (SbH,). Antimony is the link between this class of salt radicals and the basic elements, of which latter the metal bismuth is generally considered to present the greatest resemblance to antimony.

* Y represents nitrogen, phosphorus, &c.

THE LAWS OF CHEMICAL COMBINATION.

THE two great classes of elements with which the student has now become acquainted have the most powerful attraction for each other; they combine together, and in so doing are said to exercise mutual chemical affinity. When hydrogen (or any other basic element) unites with oxygen (or any other acid element), the properties which these elements possessed previous to their combination are nullified, and in the compound produced we no longer recognize the characters of its components. The extent of this neutralization varies with the nature of the combining bodies, and is more or less perfect as the elements concerned occupy more or less completely corresponding positions in their respective scales. Thus when potassium combines with chlorine, or sodium with bromine, or barium with iodine, or calcium with fluorine, or iron with sulphur, or hydrogen with oxygen, substances are produced which are said to be perfectly neutral, since in them no indication of the specific properties of the basic or acid constituent can be observed; but if a body at the summit of the basic scale be united with one low in the opposite list, or vice versâ, then the

properties of the more energetic element are found to predominate over those of the weaker, and the resulting compound exhibits either basic or acid properties: the two classes of such compounds, which are particularly abundant, are by an unfortunate nomenclature termed bases and acids, on account of their exhibiting, for the reason above given, either basic or acid properties; they are the combinations which oxygen produces with the metals, and hydrogen with the salt-radicals.

The doctrine of DEFINITE PROPORTIONS lies at the very foundation of the science of chemistry, and when any two elements are said to combine, it must be understood that they unite in welldefined ratios of weight and volume, which, to produce the same substance, are under all possible circumstances accurately the same. It matters not, for instance, whether hydrochloric acid (HCI) is produced by the simple union of its components, hydrogen and chlorine, or by any more complex mode of formation; 1 part by weight of hydrogen will always unite with 35:5 parts by weight of chlorine, while the gaseous bulks of these two quantities of matter will be equal. Again, those quantities and volumes of the other elements which are capable of replacing the hydrogen or the chlorine respectively in the typical compound hydrochloric acid (HCl) which we have chosen, are equally constant, and when once accurately determined, serve for ever as indubitable facts upon which to build the superstructure of chemical reasoning: the annexed list shows this clearly:

The term EQUIVALENT, or combining proportion, is applied to the quantities which thus enter into these most simple combinations; and in compounds more complex than these, the law of definite proportions still holds good; for if the elements exist in larger quantity, that amount is invariably found to be some