Recent observations on certain rare elements have indicated the possibility of breaking them into simpler forms, and it appears probable that in a few years our views as to the nature of the elements will undergo much change.

Atomic Theory.-We may reduce any solid, a piece of sulphur, for instance, to powder, and it would seem as if no limit existed to such division. Chemists, however, are now generally of the opinion that a limit does exist, and that every substance is made up of particles of definite size and incapable of further division. Such particles are very small, and equally hard, no matter what the nature of the mass which they constitute. They are called ATOMS (a word signifying indivisible); any mass of elementary matter consists of a collection of a greater or less number of these atoms. It is believed that the atoms are rarely, if ever, perfectly free, but associated in groups, called MOLECULES. When, therefore, we powder the sulphur, we merely separate the molecules from each other.

Molecules consisting of one kind of atoms are called elemental molecules; those containing more than one kind are called compound molecules.

Atoms and molecules are believed to be in a constant state of vibration, the rapidity of which increases with increase of temperature, and is, therefore, more rapid in the liquid than in the solid state, and still more rapid in the gaseous condition. This is known as the kinetic theory.

When a solid becomes a liquid or a liquid becomes a gas, or the reverse occurs, the molecules are not changed, but merely separated from one another. Hence the atoms in sulphur vapor are as hard and solid as those of solid sulphur, but in the vapor the pairs or molecules which they form are separated by greater distances than in the case of the solid.

The force which holds atoms together and forms them into molecules is a chemical force, and is called CHEMICAL AFFINITY. Any number of molecules of the same kind may be held together in a mass; the force that does this is called COHESION.

Atomic Weights.-Chemists have never been able to isolate or render visible atoms or molecules. Nevertheless, the progress of chemical research has developed some general principles.

Ist. That the atoms of each element have a constant and definite weight. 2d. That the atom of hydrogen is the lightest of all.

3d. That combination takes place among atoms under the action of chemical affinity.

Starting with the first two principles, numbers have been obtained which are supposed to represent the weight of each atom compared to the atom of hydrogen. These numbers are called ATOMIC WEIGHTS.

In any compound the sum of all the atomic weights is called the molecular weight. Thus, sulphuric acid is H2SO; its molecular weight is 98.

A chemical symbol is an abbreviation of the name of an element; in most cases an initial letter is used, as C for carbon, P for phosphorus. As some elements have names beginning with the same letter, proper distinction is obtained by assigning the single letter to the most common, and attaching small letters to the other initials. Thus, C stands for carbon, Ca for Calcium, Cl for chlorine, Cd for cadmium. Certain elements have different names in different languages, and for these the symbol is formed from the Latin name. Iron, for instance, is represented by Fe (ferrum); lead by Pb (plumbum); silver by Ag (argentum); potassium by K (kalium).

To express combination between elements-in other words, to express the composition of a compound body or of a molecule-the symbols are to be written together like the letters of a word. Such a collection of symbols is called a FORMULA.

The symbol, however, not only represents the element, but one atom of it. The expression CaO not only shows a compound consisting of calcium and oxygen, but also indicates that it contains a single atom of each element. CaO, shows that two atoms of oxygen are present and one of calcium. In writing these expressions certain rules are followed :

Ist. To multiply any single atom, a small number is attached to the lower right hand, as seen above, where O, indicates two of oxygen. The formula C2HO2 shows a combination consisting of two atoms of carbon, four of hydrogen and two of oxygen.

2d. To multiply several atoms by the same number, we put a large figure in front. Thus 2HCIO is equal to HClO2; that is, the large figure multiplies the whole expression.

3d. To multiply a portion of an expression, several methods are in use. We may enclose the part to be multiplied in parenthesis, and attach the proper number to the right-hand corner. Ba(NO3)2, for instance, equals BaN2O; CH(NO2)2O5 equals CСH ̧(N2O1)О. The effect of the small figure is limited to the part within the parenthesis. This method is especially adapted to multiplying symbols in the middle or at the end of a formula. To multiply the symbols at the beginning of a formula, we usually point off or punctuate the part to be affected, and place a large figure in front. Some irregularity prevails as to the particular sign used, the comma and semicolon both being employed. It is sufficient for the student to bear in mind that a punctuation mark or plus sign occurring in a formula will stop the multiplying effect of the large figure at the beginning of the expression. For instance, 2C,H,, H2N is equal to CH1H,¿N; similarly, in 2FeSO4 + HCl the letters following the plus sign are not affected by the figure 2. If we wish to carry the multiplying effect to the end of the expression, we enclose it in parentheses; thus, 2(FèSO + HCI). Here all the letters are equally influenced.

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Since the symbol of each element represents one atom, it follows that every symbol carries with it an idea of quantity. If we write HCl, the meaning is not merely that hydrogen and chlorine are in combination, but that the amounts by weight are in the proportion of the atomic weights; i. e., I (atomic weight H) to 35.4 (atomic weight Cl). When the symbol is multiplied, the weight is also multiplied. For instance, H2O represents 2 parts by weight of H to 16 of O; HgCl, represent 200 parts of mercury and 70.8 (35.4 X 2) parts of chlorine.

NOMENCLATURE.

The names of chemical compounds are regulated by a system which depends essentially upon the employment of certain terminations.

In the old division of the elements into metals and non-metals the metals were usually distinguished by the termination" um." A change of this termination into "a" indicated combination with oxygen.

Potassium (K)

becomes by oxidation, potassa (K,O); sodium (Na) becomes soda (Na2O); magnesium (Mg) becomes magnesia (MgO). As the names of many of the common metals do not end in "um" unless the objectionable Latin name is used, this rule is only of limited application. The tendency of the modern nomenclature is to make but little change in the names of the substances called metals, and the terminations about to be presented are not usually attached to bodies ending in "um," or those which we commonly call metals, such as iron, silver and zinc.

Chemical compounds which contain only two elements are called binary compounds. They are usually named by joining the names of the elements present and attaching to one of them the termination "ide." This termination may be conveniently regarded as an equivalent of the phrase "nothing else;" that is, wherever it occurs it indicates that nothing else is present except what is expressly mentioned. Potassium iodide, for instance, can contain nothing else but potassium and iodine; copper sulphide can contain nothing but copper and sulphur.

The syllable "ide" is usually attached to the members of the oxygen, chlorine, nitrogen and carbon groups, and preferably to those of the first two groups. Thus, a compound of iron and carbon is called iron carbide, but a compound of carbon and chlorine is called carbon chloride.

In many books, especially in older works, the word "of" will be found frequently used in the names of compounds. Instead of copper sulphide, we see sulphide of copper, iodide of potassium for potassium iodide. This system was introduced into chemistry by an original mistranslation of French phrases in which the word "de" occurred.

As elements may combine in several proportions, forming several different compounds, this termination ide does not suffice. The bodies Cu2O and CuO are both properly called copper oxide, because they contain only copper and oxygen, but they are different. In the same way, SO, and SO, are both sulphur oxides. The distinction is made by prefixes.

Cu2O... Copper suboxide.

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monoxide (formerly proto was used). Sulphur dioxide (formerly deut or bin was used).

Sub generally indicates deficiency; that is, that the quantity of the element to which it is attached is less than it should be. We apply the term sub especially to compounds in which a member of the oxygen or chlorine group is deficient in amount. Pb,Cl, Zn,I2, CuO would be subcompounds.

Some elements form compounds in which the proportion is as I to 1%, but as fractions are not allowed in formulæ, the whole expression is multiplied by 2, which gives the proportion 2 to 3. FeO11⁄2 becomes, therefore, Fe2O. These are called sesqui compounds, and the above expression is iron sesquioxide. The word sesqui means one and a half, and conveys the idea that the relation between the two elements is as I to 11⁄2 (2 to 3). There is no uniform method for giving names to compounds containing more than two elements. Sometimes the system is the same as that just given; all the elements are mentioned and the termination "ide" is attached. Thus KHO is potassium hydroxide, NaHO is sodium hydroxide. In other cases a portion of the compound is included under a group name, and this is joined with the names of the other elements according to the above rule. Thus KCN is not called potassium carbo-nitride, but the CN is called cyanogen, and the entire compound is called potassium cyanide.

Among the compounds containing three elements are those which we call salts. Salts are formed by the action of acids upon certain elements or their oxides. If we put zinc or zinc oxide into sulphuric acid, we get a zinc salt; in this case zinc sulphate: also by direct union of many oxides; for instance, when calcium oxide, CaO, acts upon carbon dioxide, CO2, we get calcium carbonate, CaCO3, which is a salt.

Most salts contain three elements, of which oxygen is one, and the names are made by joining the names of the other two elements and adding to them certain syllables which not only indicate the presence of oxygen, but also partly the amount. These syllables are ate and ite. The former indicates the greater quantity of oxygen. The potassium sulphate and potassium sulphite both contain oxygen, but the former (sulphate) contains the more oxygen. Sodium nitrate and sodium nitrite contain the same elements, but their composition is NaNO, and NaNO2, respectively.

It has been pointed out that the syllable ide could be regarded as equivalent to the phrase "nothing else." In the same manner, the syllables ate and ite are to be regarded as meaning "something else," generally oxygen. Thus, while in sodium sulphide but two elements are present, sodium sulphate and sulphite will contain three.

These two terminations are not sufficient. Potassium, chlorine and oxygen unite in four different proportions, forming KCIO, KCIO, KCIO,