only take place when the temperature of the substances which are to take part in them has been sufficiently raised. Thus magnesium requires to be strongly heated in air before it takes fire; once the action is started, however, the heat given out by the combustion of one part of the magnesium is sufficient to raise another part to the temperature necessary for combustion to go on, and so the change is propagated. Coal-gas only burns in air when it is raised to a bright-red heat. A jet of eoal-gas escaping into the air may be easily ignited by applying a brightly red-hot poker, but when the poker cools to dull redness it will no longer ignite the jet. A bar of metallic iron does not undergo any chemical change on exposure to dry air at ordinary temperature, but if iron in the state of very fine powder (a form in which it can easily be obtained by appropriate methods) be thrown into the air, combination at once takes place with the evolution of heat and light. When a piece of iron (say a moderately fine iron wire) is heated to redness in air, combination with the oxygen of the air takes place with the formation of a scale composed of a black oxide of iron, but the quantity of heat given out during the combination is not sufficient to propagate the combustion from particle to particle of the iron after removal of the source of | heat If, however, iron wire be raised to a red beat in an atmosphere of oxygen, it takes fire and burns with great brilliancy. The difference noticed here is due to the presence in the one case, and the aimence in the other, of the diluting nitrogen which forms nearly four-fifths of the air by volume.

There are certain chemical actions which in taking place are accompanied, not with evolution, but with absorption of heat. In such cases heat has to be supplied throughout the action, and not merely to start it. This is frequently noticed in the combination of substances which have feeble affinity for each other, and the compounds produced are

stable, or more readily break up into their continents, than those which are produced with the evolution of heat. In general terms it may be stated that the quantity of heat given out in the formation of a compound is a measure of the stality of the compound. When a given weight of magnesium unites with oxygen to form magnesia, a quite definite and measurable quantity of heat is given out. In order to separate the magnesium from the oxygen again, exactly the same quantity of heat must be supplied. In the case of those substances in the formation of which heat is absorbed, we find, as we should expect, that heat is given out during their decomposition, and that its quantity exactly that which was absorbed during their

foruation.

Chemical Notation.-For the purpose of shortly expressing the composition of chemical substances, and for representing chemical changes, chemists employ a system of notation which is in extremely common use. In the table of Atomic Weights (see ATOMIC THEORY) it will be noticed that after the name of each element is placed its symbol, which usually consists of the first, or of the first and another letter of the Latin name of the element. Each abol distinctly indicates the element which it is; intended to represent, but it must always be borne in mind that the symbol for an element is not merely A contracted form of its name, but that it stands for a definite quantity of that element, this quantity being the atomic weight expressed in terms of the it of weight employed." The unit of weight ost universally employed by chemists and scienthe men in general is the gramme (see METRIC SYSTEM, and that unit will be adopted for illustratans throughout this article. With the gramme ** unit, H stands for 1 gramme of hydrogen, Cl for 14 grammes of chlorine, O for 16 grammes of

on.

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oxygen, Mg for 24 grammes of magnesium, and so In order to represent the composition of a compound, the symbols of the various elements which occur in the compound are written side by side, and this collection of symbols is called a formula. Thus, MgO represents 40 (= 24 + 16) grammes of magnesium oxide, and HCl is 364 (=1+354) grammes of hydrogen chloride. When a compound contains more than one atom of the same element the symbol for that element is not repeated, but the number of atoms is indicated by a subscribed numeral. Thus the formula for water is written H,O, which indicates that the molecule of water contains two atoms of hydrogen and one of oxygen; and the formula for sulphuric acid is written H.SO, which indicates that the molecule of sulphuric acid contains two atoms of hydrogen, one of sulphur, and four of oxygen (besides the quantitative signification of these formula already mentioned). A number subscribed to a portion of a formula inclosed in brackets multiplies the portion so inclosed. Thus the formula Ba(NO3)2 represents one atom of barium united to twice the quantity of the group NO,, which is represented as united to one atom of potassium in the formula KNO. A number prefixed to a formula multiplies the whole of the formula that follows. Thus 2H ̧O represents twice the quantity of water represented by H.O.

Chemical symbols and formulæ are used to represent shortly chemical changes. A simple illustration of the method of using them may be given to represent the case of the burning of magnesium. The symbols for the magnesium and the oxygen entering into combination (connected by the sign +) are written on one side of what is called a chemical equation, whilst the product is written on the other side, thus:

2Mg + O1 = 2MgO.

The formula for free (or uncombined) oxygen is written Og, because a molecule of oxygen is believed to consist of two atoms (see ATOMIC THEORY). In order to represent the element magnesium, the simplest possible formula (Mg) is employed because there is no evidence for writing a more complicated one. 2Mg simply represents twice as much magnesium as Mg does.

The above equation when fully interpreted gives a great deal of information about the change which it is intended to represent. It shows that magnesium and oxygen unite with each other (under conditions which are not expressed) to form an oxide of magnesium, and that these elements are united in the compound in the proportions by weight of 24 of magnesium to 16 of oxygen; and, further, it enables us, by applying a simple and easily remembered rule, to calculate the volume of oxygen taking part in the action as well as its weight. This rule for ascertaining the volume may be conveniently stated here. From certain theoretical considerations, as well as for convenience in calculations concerning the volumes of gases, chemists write the formulæ of gaseous substances in such a way that the quantity of a gas represented by its formula, in terms of any unit of weight, shall occupy, under similar conditions of temperature and pressure, the same volume as two units weight of hydrogen. Thus, the unit being the gramme, H, represents 2 grammes of hydrogen, and 2 grammes of hydrogen at standard temperature (0° C.) and pressure (760 millimetres of mercury) occupy a volume of 22:33 litres (see METRIC SYSTEM). Similarly, the quantities in grammes of oxygen, carbonie anhydride, and nitrous oxide, represented by their respective formulæ, O, (16 x 2 = 32 grammes), CO2 (12 + 32 = 44 grammes), and N ̧O (28 + 16 = 44

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between what are called anhydrous bases and koirated bases or hydroxides. The oxide of lead, PO, is an anhydrous base (or basic oxide), whilst the compound obtained by the action of water upon calcium oxide, CaO (a basic oxide, and the only compound of calcium and oxygen known), is called a hydrated base (or hydroxide). The formation of the latter is represented by the equation

CaO + H,O = Ca(HO).

The oxides produced from non-metallic elements are very frequently acid oxides-i.e. oxides which unite with water to form the class of bodies called Ands (q.v.). The oxides themselves are often called acid anhydrides, whilst the compounds produced by the action of water upon them are called acids, or hydrogen salts. When phosphorus burns in air, phosphoric anhydride, POs, is obtained. This is a white solid substance which unites with water with the evolution of much heat to form a solution of metaphosphoric acid, or hydrogen metaphosphate:

There are a few acids known which do not contain oxygen, and are not obtainable by the combination of an oxide with water. Examples are I hydrochloric acid, HCl, hydrobromic acid, HBr, and hydrocyanic acid, HCN. These are also called hydrogen chloride, bromide, and cyanide respectively.

The two classes of substances, bases and acids, are nearly related to the very large class of salts. A salt is a compound which can be obtained, amongst other ways, by the action of an acid upon a hase, water being almost invariably eliminated at the same time; and just as we saw that the properties of two elements are totally different from those of the compound formed by their combination, so we find that in the formation of a salt the properties of both acid and base to a great extent or altogether become neutralised and disappear.

If to a solution in water of potassium hydroxide, KHO which is a powerful base), we add a suffi eient quantity of nitric acid, HNO,, that is until the liquid on thoroughly mixing does not possess either the acid or the alkaline reaction, we obtain a solution in water of potassium nitrate (saltpetre), and nothing else the water eliminated in the action simply mixing with that which is already present:

KHO + HNO3 = KNO3 + H2O.

Acids have already been mentioned as hydrogen salta. The above equation shows how hydrogen nitrate is exactly comparable with potassium Eitrate an atom of potassium taking the place of an atom of hydrogen and a characteristic of all hydrogen salts, or acids, is that they contain hydro2, which is capable of removal and of having its pare thus taken by an equivalent quantity of Ather metal. In the example above mentioned mery 1 part by weight of hydrogen has its place taken by 39 parts by weight of potassium. These Juantities of hydrogen and of potassium are equiv. ent. both being capable of uniting with the group NO, This group is an example of what is called a compound radical-i.e. a group of ements which is capable of going as a whole tagh a series of changes. Acids which contain in their molecule one atom of hydrogen replaceable In another metal are called monobasic acids, Sitre acid is thus a monobasic acid, whilst sul Huvir acid, H,SO, is dibasic, orthophosphoric acid, HP, is tribanc, and so on.

Bases, likewise, are sometimes spoken of as ad, diacid, triacid, and so on, according as molecule of the base requires one, two, three,

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&c. molecules of a monobasic acid (as nitric acid) to form what is called a normal salt, that is, a salt in which all the replaceable hydrogen has been replaced by another metal. Thus potassium hydroxide, KHO, is a monacid base; calcium hydroxide, or slaked lime, Ca(HO), is diacid; bismuth hydroxide, Bi(HO), is triacid, and so on. Equations may make this clearer (see the equation above for a monacid base):

Ca(HO)2 + 2HNO3 = Ca(NO3)2 + 2HO : Bi (HO) + 3HNO3 = Bi (NO3); + 3H2O. Salts are formed in many cases by the replacement of only a part of the replaceable hydrogen of a hydrogen salt by another metal. Such are called acid salts, and KHSO, is an example. This salt, KHSO4, may be looked upon as intermediate be tween the acid, H.SO, and the normal salt, KSO4.

Many salts are known which may be looked upon as bases which have their basic character only partially neutralised by an acid. Such salts are called basic salts, and as examples may be mentioned BIONO, and Pb(OH)NO,. The former is intermediate between the normal nitrate, Bi(NO3)3, and the oxide, BiO, the latter between the nor mal nitrate, Pb(NO3),, and the hydrate, Pb(OH),. Such basic salts are often produced by the action of water upon the normal salts, as, for instance, in the case of the basic bismuth nitrate:

Bi (NO), H,O= 2HNO, + BiONO, Salts are looked upon as being composed of metal and salt radical, the latter name being given to all of the salt that is not metal. Thus SO, is the salt radical of the sulphates, NO, the salt radical of the nitrates, &c. This way of looking at salts arises from the phenomena observed when salts are decomposed by Electrolysis (q.v.), metal and salt radical being the primary products of decomposition.

Chemical Nomenclature.-Chemists endeavour to make the nomenclature of compound substances as systematic as possible, and a certain amount of system has even been introduced into the nomenclature of the elements themselves. The oxides of the metals are named after the metal which they contain, as magnesium oxide, MgO; aluminium oxide, ALO,; and the series of salts derivable from these oxides are similarly named after the metal. Thus MgCl, is magnesium chloride, and Al(SO4)3 is aluminium sulphate. When a metal formis more than one basic oxide, adjectival terminations are employed to distinguish these; thus the two basie oxides of iron are named ferrous and ferric oxides (FeO and FeO3) respectively, and correspondingly there are ferrous and ferric salts. FeSO, is ferrous sulphate; Fe,Cl, is ferric chloride. Acid salts and in general safts which contain more than one metal are named after the metals which they contain, the compound radical NH (ammonium; see AMMONIA) being regarded as a metal for purposes of nomenclature. Thus, KHSO is potassium hydrogen sulphate, whilst HNANH,PO, is hydrogen sodium ammonium orthophosphate.

The nomenclature of non-basic metallic oxides has been rendered systematic by the use of names descriptive of the number of atoms of metal and of oxygen contained in the oxide, as, for instance, trimanganic tetroxide for Mn,0,. A considerable number of non-basic oxides, as BaO., PbO4. MnO2, &c., are somewhat less systematically called per oxides.

The acid anhydrides, which, as has already been stated, are oxygen compounds or oxides of the nonmetallic elements, are named after the elements of which they are oxides. As there are frequently two or more such acid anhydrides derived from one

ehloride will be precipitated, and only sodium nitrate will remain in solution. The action may be represented by an equation:

NaCl + AgNO, = NaNO, + AgCl (precipitate). The action of sulphuretted hydrogen on many metallic solutions illustrates double decompositions in which the action is complete, as,

HgCl + HS = HgS + 2HCl,

where the mercuric sulphide formed is insoluble in water, and is consequently obtained as a pre

cipitate.

In connection with the subject of double decomposition the bearing of the law of Richter (already mentioned in the historical sketch) may be illus trated. Looking at the quantitative signification 1 of the following equations,

KCl + AgNO, = AgCl + KNO3,

NaCl + AgNO, = AgCl + NaNO3,

we see that the quantity of chlorine which was united with 39 parts by weight of potassium or 23 of sodium to form a salt is exactly the quantity required to form a salt with 108 parts by weight of ver, whilst, similarly, the quantity of the group NO, which was united to these 108 parts by weight of silver is exactly the quantity required to form a salt with 39 parts by weight of potassium or 23 of ium. The same holds good generally for double decompositions.

Another very important kind of chemical change is the displacement of one element in a compound by another. Chlorine, for instance, displaces the dine in potassium iodide and takes its place:

2KI + Cl2 = 2KCl + I.

The greater affinity of potassium for chlorine than for odine is the explanation given of this displacement Displacement of one metal by another is a familiar phenomenon, although the chemistry of what is taking place may not be familiar to all who have seen it. When a piece of bright iron or steel, as a key or the blade of a knife, is dipped into an aufulated solution of cupric sulphate (blue vitriol), a reddish deposit of metallic copper is formed a most immediately upon the surface of the metal. Thas copper is derived from the cupric sulphate solution; but what is not manifest from observation alone, is that at the same time an equivalent quantity of iron is dissolved away and goes into sation as ferrous sulphate. The action is,

CuSO, + Fe = FeSO, + Cu.

The whole of the copper would eventually be separated from the solution in the metallic state if enough iron were present, and for every 63 parts of copper precipitated 56 parts of iron would go into

secution.

Inorganic and Organic Chemistry.-The whole nject of chemistry has been divided into two great divisions, named respectively inorganic and organic. Made originally to separate from each other the chemistry of purely mineral substances, and that of substances of animal or table origin, which were at the time suppe to be capable of formation only as products of vital processes, this subdivision is retained still manly as a matter of convenience. The division of organic chemistry is sometimes spoken of now as the chemistry of the compounds of carbon; but 1 is not a very strict definition, as many carbon pands occur in nature as purely mineral substanres, and having really no connection with ane chemistry, such as numerous mineral carate As has been already stated, it is mainly for convenience that the consideration of the ority of the compounds of carbon is taken as a rate branch, not because of any difference in

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the chemical principles involved, but really on account of the very great number of these compounds, and of the great complexity of many of them.

It is in the domain of organic chemistry that the study of the constitution of substances has been most diligently prosecuted, and with the greatest amount of apparent success. The graphic formula which chemists assign to acetic acid (to take a simple example) is,

H

H -C-O-H. This formula is adopted in order

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to express a number of ideas concerning the supposed mode of arrangement of the atoms in acetic acid, deduced from the study of its formation, its decompositions, and the action upon it of various substances. The known facts find suitable expression in the formula, and there is no observation yet made as to the chemical relations of acetic acid which is at variance with the constitution indicated by it. It would not be possible here to quote evidence in favour of a particular constitution for any substance, but it may be stated generally that chemists endeavour to fix the constitution of the simplest compounds on the firmest possible basis, and, in passing from the simple to the more complex, to make secure every step.

The tetravalent character of the carbon atom, and the great facility with which carbon atoms enter into combination with other carbon atoms and with the atoms of other elements, give their impress to the whole of organic chemistry. The graphic formulæ of organic substances amply illustrate the former, whilst the syntheses of a long array of simple and complex organic compounds as amply illustrate the latter.

A certain amount of knowledge of chemistry is eminently useful in almost every walk of life. An intelligent knowledge of the chemistry involved in the processes of the kitchen, the dairy, the dyehouse, the farm, or the manufactory, places the possessor engaged in any of these processes on a different level from the rule-of-thumb worker, who is as ignorant of the reason for adopting a particular method as he is of the properties of the materials he employs. Technical chemistry deals especially with the application of the principles and processes of chemistry to the arts and manufactures, and it is to those who are engaged in manufactures of almost every kind that a knowledge of chemistry is a particular advantage. It is not a question of expediency alone, but one of absolute necessity that a technical education, including chemistry as one of its principal subjects, should form not the least important part of the equipment for his work of any artisan who is to excel in his employment in intelligence and skill.

In connection with this article should be read the article ATOMIC THEORY, which is to a certain extent supplementary to this. ANIMAL CHEMISTRY and VEGETABLE CHEMISTRY are separately treated. The reader is also referred to the description of each element under its name, to those of the acids under their names, and to the following as amongst the most important of the large number of chemical articles throughout this work: