DOUBLE OR PRODUCT-VOLUME.

207 twice the unit-volume. This doubled volume is often called the normal or product-volume of a compound gas. Since the combining weight of a compound gas or vapor occupies two unitvolumes, it is obvious that the weight of one volume, which is the specific gravity of the gas or vapor, is deduced from the combining weight by dividing the latter by two. The specific gravity of a compound gas or vapor is, therefore, one-half its combining weight.

261. Molecular condition of elementary gases.-Bearing in mind our definitions of atom and molecule (§§ 38, 39), let us inquire what inferences concerning the molecular condition of simple gases in a free state can be legitimately drawn from our knowledge of the molecular condition of compound gases. To give definiteness to our conceptions, let us assume the unit-volume of the elements to be one litre; the product-volume of a compound will then be two litres. Two litres, the product-volume, of chlorhydric acid gas are made up of one litre of hydrogen and one litre of chlorine, united without condensation, and each molecule of chlorhydric acid must contain at least one atom of hydrogen and one of chlorine. In these two litres of chlorhydric acid there must be some definite number of molecules; the number is, of course, indeterminable; but let us assign to it some numerical value, say 1000, in order to give clearness to our reasoning. One litre of chlorhydric acid will then contain 500 molecules, and since equal volumes of all gases, whether simple or compound, are assumed to contain, under like conditions, the same numbers of molecules (§ 39), one litre of hydrogen or of chlorine will also contain 500 molecules. But the one litre of hydrogen and the one litre of chlorine, which, by uniting, produced 2 litres =1000 molecules of chlorhydric acid, must each have contained 1000 atoms of hydrogen and of chlorine respectively, for each molecule of chlorhydric acid demands an atom of hydrogen and an atom of chlorine. The litre of hydrogen, or of chlorine, then, contains 500 molecules, but 1000 atoms,-each molecule of the simple gas being made up of two atoms of the single element, just as each molecule of the compound gas under review is composed of two atoms, one of hydrogen and one of chlorine. It is clear that this train of reasoning is independent of the particular numerical value assumed as the number of molecules in two litres of chlor

208

MOLECULES OF ELEMENTARY GASES.

hydric acid. If, therefore, the molecule of chlorhydric acid is represented by the formula HCl, and the diagram

there is good reason to assign to free hydrogen and free chlorine the formulæ HH and CIC1, and to represent the constitution of all uncombined gases by such diagrams as

H+

H = HH

Cl + Cl = CICI

Upon these models the molecular formulæ of all the elements with which we have become acquainted might readily be written. It is only in a free state that the elementary gases and vapors are thus conceived to exist as molecules; when they enter into combination, it is by atoms rather than by molecules. An atom of hydrogen unites with an atom of chlorine; three atoms of hydrogen combine with one of nitrogen.

If this view of the molecular structure of free elementary gases and vapors be correct, perfect consistency would require that no equation should ever be written in such a manner as to represent less than two atoms, or one molecule, of an element in a free state as either entering into or issuing from a chemical reaction. Thus instead of H+0=H20, N+3H=NH ̧, HCl+Na=NaCl+H, it would be necessary to write

2HH

+

00 = 2H,0,

NN + 3HH = 2NH,, 2HCl + NaNa = 2NaCl + HH.

We have not heretofore conformed to this theoretical rule, and do not propose to do so in the succeeding pages, and this for two reasons:-first, because many equations, representing chemical reactions, must be multiplied by two in order to bring them into conformity with this hypothesis concerning molecular structure; the equations are thus rendered unduly complex; secondly, because, in undertaking to make chemical equations express the molecular constitution of elements and compounds, as well as the equality of the atomic weights on each side of the sign of equality, there is imminent danger of taking the student away from the sure

ground of fact and experimental demonstration, into an uncertain region of hypotheses based only on definitions and analogies. The symbol Na represents 23 proportional parts by weight of the metal sodium; of the molecular symbol NaNa, the most that can be said is, that some strong analogies justify us in assuming, for the present, in default of any experimental evidence on the subject, that a molecule of free sodium gas, if we could get at it, would be found to consist of two least combining parts by weight of sodium. We know as much, at least, of the molecular structure of sodium as we do of four-fifths of the recognized chemical elements. For the present, the biatomic structure of the molecule of a simple gas or vapor in the free state must take place, in an elementary manual, as an ingenious and philosophical hypothesis, rather than as a general and indubitable fact.

CHAPTER XVI.

PHOSPHORUS.

262. Phosphorus occurs somewhat abundantly and very widely diffused in nature. It is never found in the free state, but almost always in combination with oxygen and some one of the metals. The most abundant of its compounds is phosphate of calcium; small quantities of this mineral are found in most rocks and soils, and in several localities it occurs in large beds. Phosphate of calcium is the chief mineral constituent of the bones of animals; it contains one-fifth of its own weight of phosphorus. The proportion of phosphorus present in most of the ordinary rocks, and in the soils which have resulted from their disintegration, is usually very small, and phosphorus would be an exceedingly costly substance if we were compelled to collect it directly from this source; but it so happens that the phosphorus-compounds are important articles of food for plants and animals, and it is easy to obtain through their intervention the phosphorus which was before widely diffused, but has been by them concentrated.

P

210

ORDINARY PHOSPHORUS.

Growing plants seek out and collect the traces of phosphoruscompounds which exist in the soil; the herbivorous animals in their turn consume the phosphorus which has been accumulated by the plants, and from the bones of animals chemists and manufacturers derive the phosphorus of which they stand in need.

Like oxygen and sulphur, phosphorus occurs in several distinct allotropic modifications. Of these, the best-known are called respectively ordinary phosphorus and red phosphorus.

263. Ordinary phosphorus, when perfectly pure, is a transparent, colorless, wax-like solid of 1.8 specific gravity, which, when freshly cut, emits an odor like garlic, though under ordinary conditions this odor is overpowered by the odor of ozone, which, as has been previously stated (§ 164), is developed when phosphorus is exposed to the air. It unites with oxygen readily, even at the ordinary temperature of the air, and with great energy at somewhat higher temperatures (above 60°); when in contact with air it is all the while undergoing slow combustion.

Exp. 106. Thoroughly wash a piece of phosphorus by rinsing it in successive large quantities of water; place it, for a moment, upon a sheet of filter-paper, in order that a portion of the water adhering to it may be removed, then lay it upon a clean porcelain capsule, and at short intervals press against it a slip of blue litmus paper. In a very few moments the color of the paper will be changed to red; for the products of the oxidation of phosphorus are acid, and they are formed with great rapidity.

If the temperature of the slowly burning phosphorus be slightly increased in any way, the mass will burst into flame and be rapidly consumed. On account of this extreme inflammability, phosphorus must always be kept under water; it is best also to cut it under water, lest it become heated to the kindling-point by the warmth of the hand or by friction against the knife. When wanted for use, the phosphorus is taken from the water and dried by gently pressing it between pieces of blotting-paper.

Phosphorus must always be handled with great caution; for when once on fire, it is exceedingly difficult to extinguish it, and, in case it happens to burn upon the flesh, painful wounds are inflicted, which are exceedingly difficult to heal. Whenever phosphorus is cut or broken, care must likewise to taken that no small

fragments of it fall unobserved into cracks of the table or floor, where they might subsequently take fire.

Exp. 107.-Nip a piece of phosphorus, as large as a small pea, between two bits of wood, in such manner that a part of the phosphorus shall project below the wood; rub the phosphorus strongly upon a sheet of coarse paper; it will take fire at the temperature developed by the friction.

264. On account of this easy inflammability by friction, phosphorus is extensively employed for making matches. The matter upon the end of an ordinary friction match usually contains a little phosphorus, together with some substance capable of supplying oxygen, such as red-lead, black oxide of manganese, saltpetre, or chlorate of potassium. The phosphorus and the oxidizing agent are kneaded into a paste made of glue or gum, and the wooden match-sticks, the ends of which have previously been dipped in melted sulphur, are touched to the surface of the phosphorized paste, so that the sulphured points shall receive a coating of it. The sulphur serves merely as a kindling material which, as it were, passes along the fire from the phosphorus to the wood. By rubbing the dried, coated point of the match against a rough surface, heat enough is developed to bring about chemical action between the phosphorus and the oxygen of the other ingredient, combustion ensues, the sulphur becomes hot enough to take on oxygen from the air, and finally the wood is involved in the play of chemical force.

Exp. 108.-Put a piece of phosphorus, as big as a grain of wheat, upon a piece of filter-paper, and sprinkle over it some lampblack or powdered bone-black. The phosphorus will melt after a time, and will finally take fire. As will be more fully explained hereafter, under carbon, the porous, finely divided lampblack has the power of absorbing and condensing within its pores much oxygen from the air; heat is developed by the act of condensation, and, at the same time, oxygen is brought into very intimate contact with the phosphorus, particularly with the vapor of phosphorus which is condensed by the lampblack together with the oxygen, so that chemical action soon results, and ultimately fire. Both the lampblack and the paper are bad conductors of heat; they prevent the phosphorus from losing the heat developed by the condensation and by the slow action of oxygen.

It is remarkable that when dry phosphorus, in very thin slices, is