orbit. In the case of the heavenly bodies, the force which regulates the movements of the individual members of the system amongst themselves is the same force that controls the motion of the united system, namely, gravitation. What is the precise relation, or difference, if any, between the forces which control the movements of molecules, and those which operate between the atoms of the molecule, is not known; but as the effects produced are different, the latter force is distinguished by the name of chemical affinity. Any change which matter undergoes, in which the integrity of the molecules is not destroyed, is regarded as a physical change; while any change which arises from an alteration in the structure of the molecule is a chemical change. For example, the molecules of water consist of three separate atoms, one of oxygen and two of hydrogen; any change which water can be made to undergo, in which these three atoms still remain associated together as the molecule, is a physical change. The water may be converted into ice, or it may be changed into steam; but these alterations still leave the molecules intact-the three atoms still remain united as an unbroken system, and so long as this is the case chemical change has not taken place. Suppose now the molecules of water are heated to a much higher temperature than that which is necessary to convert the water into steam, by passing electric sparks through the steam. It will then be found that a very different kind of change has come over the substance. The steam, after being so heated, no longer condenses to water again when cooled; it has been changed into a gas which can be bubbled through water and collected in an inverted vessel filled with water standing in a pneumatic trough, and if a flame be applied to this gas a sharp explosion takes place. The change in this case is a chemical change, for the integrity of the molecules of water has been destroyed. The two atoms of hydrogen have become detached from the oxygen atom, and the original triune structure of the system is destroyed. Atoms are therefore defined as the smallest particles of matter which can take part in a chemical change. し CHAPTER II ELEMENTS AND COMPOUNDS THERE are certain molecules in which all the atoms present are of the same kind, and there are other molecules which are composed of atoms which differ from one another. Thus, in the substance sulphur, all the atoms composing the molecules are alike; while in water, as already mentioned, there are two distinct kinds of atoms in the molecule. Matter, therefore, is divided into two classes, according as to whether its molecules are composed of similar or of dissimilar atoms. Molecules consisting of atoms of the same kind are termed elementary molecules, and substances whose molecules are so constituted are known as elements; molecules, on the other hand, which contain dissimilar elements are called compound molecules, and substances whose molecules are thus composed are distinguished as compounds. Sulphur, therefore, is an element, and water is a compound. It will be evident that in the case of elementary molecules, whatever processes they may be subjected to, only one kind of matter can be obtained from them; while in the case of compounds, the molecules consisting of dissimilar atoms, as many different kinds of matter can be obtained as there are different atoms present. By appropriate means the atoms of hydrogen and oxygen in water molecules can be separated, and two totally different kinds of matter, namely, hydrogen and oxygen, can be obtained from this compound. At the present time there are about seventy substances known to chemists which are believed to be elements. In the history of the science it has frequently happened that substances which were considered to be elements have proved, when subjected to new methods of investigation, to be in reality compound bodies: thus, prior to the year 1783, water was thought to be an elementary substance; it was indeed regarded as the very type of an element, until Cavendish and Lavoisier proved that it was composed of two entirely different kinds of matter. In the year 1807 Sir Humphry Davy showed that the substances known as potash and soda, which were believed to be elements, were in reality compound bodies, and he succeeded in separating the constituent atoms in the molecules of these substances, and in obtaining from them two essentially different kinds of matter. It is therefore quite possible, perhaps even probable, that some at least of the forms of matter which are now held to be elements may yet prove to be compound bodies. On the other hand, the list is from time to time extended by the discovery of new elements. Thus during the last few years at least five new members have been added to the number. The number of compounds is practically infinite. The elements are very unequally distributed in nature, and are of very different degrees of importance to mankind. Some are absolutely essential to life as it is constituted, while others might be blotted out of creation without, so far as is known, their absence being appreciated. The following thirty elements include all the most important (for the complete list see page 22) :— On account of certain properties common to a large number of the elements, and more or less absent in others, properties which are for the most part physical in character, the elements are divided into two classes, known as metals and non-metals. The metals generally are opaque, and their smoothed surfaces reflect light to a high degree, thus giving them the appearance known as metallic lustre. They also conduct heat and electricity. Gold, silver, copper, iron, are metals; sulphur, bromine, oxygen, phosphorus, are non-metals. These two classes, however, gradually merge into one another, and certain elements are sometimes placed in one division and sometimes in the other, depending upon whether the distinction is based more upon their physical or their chemical properties: thus, the element arsenic possesses many of the physical properties of a metal, but in its chemical relations it is more allied to the non-metals; such elements as these are often distinguished by the name metalloids. The following list embraces all those elements which by common consent are regarded as non-metals and metalloids, including the recently discovered elements of the argon group, which are here printed in italics : atom. Krypton. The number of atoms which compose the various elementary molecules is not the same in all cases: thus in the elements sodium, potassium, cadmium, mercury, and zinc, the molecules, when the elements are in a state of vapour, consist of only one The same is true also of the newly discovered elements in the last column. The molecules of all these substances are single particles of matter. The terms molecule and atom, therefore, as applied to these elements, are synonymous. Such molecules as these are called mono-atomic molecules. In many cases elementary molecules consist of two atoms; such is the case with the elements hydrogen, bromine, chlorine, oxygen, nitrogen, and others. Elementary molecules of this twin or dual nature are known as di-atomic molecules. Only one instance is known in which an elementary molecule consists of a trio of atoms, namely, the molecule of ozone, which is an aggregation of three oxygen atoms. This molecule is said to be tri-atomic. In two cases, namely, arsenic and phosphorus, the molecules are composed of a quartette of atoms, and these elements, therefore, are said to form tetr-atomic molecules. In a large number of instances the atomic constitution of the molecule of the elements is not known. These terms, mono-atomic, di-atomic, &c., are applied exclusively to molecules of elements, and are not used in reference to compounds, where the molecules are composed of dissimilar atoms. Mechanical Mixtures.-When molecules of different kinds of matter are brought together, one of two results may follow: either they will merely mingle together without losing their identity, that is to say, the atoms composing the individual molecules will still remain associated together as before, or the atoms in the molecules of one kind will attach themselves to certain atoms present in molecules of another kind to form still different molecules; in other words, there will be a redistribution of the atoms, whereby different systems or molecules are produced. In the first case the result is said to be a simple or mechanical mixture, in the second it is the formation of a chemical compound. In a simple mixture the ingredients can be again separated by purely mechanical methods; and as the properties of a substance are the properties of the molecules of that substance, it follows that if the integrity of the molecules is not broken, the properties of a mechanical mixture will be those of the ingredients. For example, oxygen is a colourless gas without taste or smell; hydrogen also is a colourless gas without taste or smell: when these two gases are mixed together, the mixture is gaseous, is colourless, and tasteless, and, being only a mixture, the molecules of one gas can be readily sifted away from the other. Again, charcoal is a black solid, insoluble in water; sulphur is a yellow solid, also insoluble in water; nitre is a white solid, readily dissolved by water: when these three substances are finely powdered and mixed together, the result is a mechanical mixture, which is solid, and which is dark grey or nearly black in colour. If this mixture be placed in water, the nitre is dissolved away and the charcoal and sulphur are left. When, however, the integrity of the molecules is disturbed, when, by bringing together molecules of different substances, a rearrangement of the atoms takes place, resulting in the formation of new molecules, then it is said that chemical action has taken place. Chemical action, therefore, always results in the formation of new molecules-new molecules which are endowed with their own special properties, differing often in the most remarkable and quite inexplicable manner from those of the original molecules. One or two examples may be quoted in order to illustrate this extraordinary modifying effect of chemical action. The two colourless gases, oxygen and hydrogen, when simply mixed together, give rise, as already mentioned, to a colourless, gaseous mixture, in which the dual molecules of hydrogen and the similarly constituted oxygen molecules move about freely amongst |