DESCRIPTIVE CHEMISTRY.

A COMPLETE table of the elements, their valencies, atomic weights and symbols, will be found at the end of the book. The following is a summary of the important groups :

I. The Potassium Group includes hydrogen, lithium, sodium, potassium, rubidium, cæsium and silver. They are positive monads, and have high affinity for members of the oxygen and chlorine groups. With oxygen they produce (except hydrogen and silver) powerful corrosive bases called the alkalies, and on this account are sometimes called the alkali metals. Hydrogen and silver are the only ones that occur free in nature.

2. The Chlorine Group includes fluorine, chlorine, bromine and iodine. They are negative monads, and are the only elements which form salts without the aid of some member of the oxygen group. For this reason they have been called the HALOGENS, a word meaning "salt-formers."

3. The Oxygen Group includes oxygen, sulphur, selenium and tellurium. They are negative dyads, and possess the power of forming, with many elements, basic or acid compounds, according to the proportion in which they are combined.

4. The Nitrogen Group includes boron, nitrogen, phosphorus, arsenic, antimony, bismuth and gold. They are of uneven valency, triads or pentads; their electrical relations are intermediate in character, neither strongly positive nor strongly negative.

5. The Carbon Group includes carbon, silicon, titanium, tin and some rarer elements. They are tetrads, and, like the nitrogen group, their relations are intermediate. Boron is sometimes classed here, but it is best placed in the nitrogen group. Platinum may be included here.

6. The Calcium Group includes calcium, barium, strontium and lead. They are positive dyads, and form oxides which are slightly soluble in water, but much less caustic or corrosive than the alkalies proper, and are often called alkaline earths. Their sulphates, carbonates and phosphates are practically insoluble in water.

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7. The Zinc Group includes zinc, magnesium, cadmium and beryllium. They are never found free, but are tolerably easily reduced from their compounds. They are positive dyads, and each form a definite oxide which is insoluble in water, not caustic, but forming well marked salts.

8. The Iron Group is positive, and includes aluminum, iron, manganese, chromium, nickel, cobalt and probably several other elements the chemistry of which is not well known. They are not found in the metallic state, except in small quantity. Most of them form two sets of compounds, *acting in one as dyads, in the other as double tetrads. Several form wellmarked acid anhydrides.

9. The Copper Group includes copper and mercury, positive dyads, resembling each other in the power of forming two sets of compounds, in one of which they are in the unsaturated condition. In this condition they form chlorides insoluble in water, and are thus partly related to silver.

10. The Platinum Group.-A number of elements which are found in association with platinum are usually grouped together under the name of platinum metals. These are palladium, iridium, rhodium, ruthenium and osmium.

Unclassified Elements.-Some of the elements are either so rare that their relations have not yet been satisfactorily studied, or their properties are such as to render it impossible to classify them satisfactorily under any system.

Potassium Group.-The potassium group proper includes potassium, sodium, lithium, rubidium, cæsium. They are positive monads, of high affinities. Their compounds are nearly all soluble in water. Their oxides and hydroxides are powerfully corrosive, and are known as the caustic alkalies. Hydrogen and silver, being positive monads, are also classed in this group, although they differ from the rest in some points.

HYDROGEN.

Hydrogen, H, I, exists in water and all organic substances.

It is prepared by the action of electricity on water or dilute acids; also by the action of certain elements on water or acids. With acids the action generally occurs without the aid of heat; with water, sodium and potassium act in the cold; iron, magnesium, zinc, etc., require a high temperature.

Zn + H2SO1 = ZnSO4 + H2.

The last method is generally used in the laboratory.

Hydrogen is also evolved readily by the action of sodium hydroxide on aluminum.

Pure hydrogen is a colorless, tasteless and odorless gas. It is the lightest body known, a litre weighing 0.08961 grm. 100 cubic inches weigh 2.14 grains. It can be liquefied only by intense cold and pressure. It will burn in air or oxygen, forming water.

Hydrogen, though not poisonous, will not sustain life; small quantities, when pure, can be inhaled without danger.

Hydrogen is a positive monad, and a standard for valency, atomic and molecular weight and density. It combines with many elements. It is the essential element of acids.

Water, H2O.-When two volumes of hydrogen and one volume of oxygen are combined, complete condensation takes place and water is formed.

Water exists abundantly, not only collected in masses, as in rivers, lakes and seas, but in combination with many substances and in a state of mixture with inorganic and organic bodies. Air almost always contains some water. Some living structures, as succulent fruits, jelly fish, etc., consist almost entirely of water. In natural conditions water is never pure. The matter

ordinarily dissolved varies from five to thirty grains to the gallon. When the quantity greatly exceeds this, and especially when peculiar substances, such as iron or sulphur, are present, it constitutes a mineral water. water is very rich in mineral substances.

The most important varieties of mineral waters are—

Sea

Alkaline or carbonated waters, containing various carbonates in solution, generally with a quantity of free carbonic acid.

Chalybeate waters, containing iron, generally as ferrous carbonate, with excess of carbonic acid.

Sulphur waters, containing sulphureted hydrogen and other sulphides. Acid waters, containing some of the stronger acids.

Saline or aperient waters, having large amounts of chlorides and sulphates.

Water combines with many bodies. There are two principal classes of these compounds. In one the water seems to unite as such with the other substance, in the other class the molecule of water is broken up.

Of the first kind of combination instances are seen in common crystals. The blue crystals sold as copper sulphate have the composition CuSO4, 5H2O. Water that is in this way part of a molecule, and essential to a crystalline form, is called WATER OF CRYSTALLIZATION. Substances that do not contain it in such a state of combination are said to be ANHYDROUS. Water of crystallization is usually easily driven out by heat.

The second form of the chemical action of water is seen if we mix water with quicklime; a violent action ensues, and the compound CaH2O, results. A considerable number of oxides are capable of uniting thus with water and forming bodies known as hydroxides.

The oxides which, by addition of water, produce hydroxides are called ANHYDRIDES. By subtracting H2O from any hydroxide we may reproduce the corresponding anhydride. Acids in this way furnish anhydrides :

If the acid contains but one atom of hydrogen, we must, of course, double the formula before subtracting. Hence

The term anhydride generally refers to those bodies which yield acids by addition of water. Those which yield hydroxides capable of neutralizing acids are generally called bases.

Hydrogen Dioxide, hydrogen peroxide, H2O2, H—0—0—H.—This body, sometimes called oxygenated water, is prepared by liberating oxygen in the presence of water, or when certain highly oxidized bodies are dissolved, as in the following reaction:

The presence of a considerable excess of water is necessary to the success of this reaction. (See Ozone.) It is a colorless, transparent, oily liquid, nearly one-half heavier than water; it is without odor, has a bitter taste, blisters the skin and bleaches organic colors. It is decomposed by heat and by many chemical substances, often explosively. The preparation of the concentrated liquid is difficult, but a somewhat dilute solution can be easily made and kept, and is now a commercial article, being used for bleaching hair, and as a disinfecting and oxidizing agent.

POTASSIUM.

Potassium, K, 39, occurs in many rocks and soils and in the ashes of land plants, also as nitrate and chloride. It is quite soft, quickly tarnishes in the air, and decomposes water rapidly, the escaping hydrogen being so highly heated as to take fire, burning with a purple flame, due to the presence of potassium. Specific gravity, 0.865. It is highly positive, and forms several oxides.

Potassium Hydroxide, KHO, Caustic Potassa, is made by boiling potassium carbonate with slaked lime.

CaH,O, + K,CO, 2KHO+ CaCO3.

=

Caustic potassa is a white, powerfully alkaline solid.

Potassium Carbonate, K,CO, Salt of Tartar.-This is extracted from the ashes of land plants. Pure potassium carbonate is white, alkaline and moderately corrosive.

Acid Potassium Carbonate, KHCO3, Salæratus, is a white crystalline body, and is used in effervescing mixtures, but acid sodium carbonate has of late years substituted it to a great extent. It is often called bicarbonate. Potassium Sulphate, K2SO, forms hard crystals, not very soluble in cold water.

Acid Potassium Sulphate, KHSO4, is sour and strongly acid to test paper. It is often called bisulphate.

Potassium Nitrate, KNO,, Nitre Saltpetre, is found on the surface of