Hence, each atom of hydrogen chloride is composed of half an atom of hydrogen united with half an atom of chlorine. But, by definition, an atom of an element is not separated into parts when it interacts chemically with atoms of other elements or compounds. Hence either Gay-Lussac's generalisation is wrong, or the Daltonian definition of the elementary atom must be modified. In 1811 the Italian naturalist Avogadro modified the 283 Daltonian atomic theory by introducing the conception of two orders of small particles, the molecule and the atom. The molecule of an element or compound, said Avogadro, is the smallest mass of it which exhibits the characteristic properties of that element or compound. The molecule, he said, is formed of smaller parts; these are atoms. The atoms which form the molecule of an element are all of one kind; the atoms which form the molecule of a compound are of two, or more, different kinds. Avogadro's conception of the structure of matter applied to the case of water asserted that, if the separation of a quantity of water could be carried far enough, we should at last come to very minute particles each of which would exhibit the properties of water; but if these particles were separated into parts we should no longer have particles of water, but particles some of which would exhibit the properties of hydrogen and some the properties of oxygen. Similarly, if the separation into parts of a quantity of hydrogen could be carried far enough, the hypothesis asserted that we should at last come to very minute particles each of which would exhibit the characteristic properties of hydrogen; but if these particles were separated into parts we should no longer have particles of what we know as hydrogen, but particles more or less unlike hydrogen, yet each the same as all the others. In other words, the Avogadrean conception of the structure of elements and compounds asserts, (1) that a quantity of a compound, or of an element, consists of a vast multitude of minute particles each of which possesses the characteristic properties of the compound, or of the element; these particles are called molecules; (2) that each of these molecules itself consists of a fixed number of yet smaller particles; these smaller particles are called atoms; (3) that the properties of the atoms which form the molecule of a compound are very different from the properties of the molecule itself; (4) that the properties of the atoms which form the molecule of an element are also different from the properties of the molecule of that 284 285 element, but that inasmuch as the atoms which form the molecule of an element are all of one kind, and are only more minute portions of the same kind of matter as the molecule itself, there is not so marked a difference between the properties of these atoms and the properties of the molecule formed by their union, as there is between the properties of the atoms of the different elements which form a compound and the properties of the molecule of that compound. Avogadro modified the generalisation of Gay-Lussac, and gave it the following form : Equal volumes of gaseous elements and compounds, measured at the same temperature and pressure, contain equal numbers of molecules. Let us apply this generalisation to the combination (1) of hydrogen and chlorine to form hydrogen chloride, (2) of hydrogen and bromine to form hydrogen bromide. In each case we shall suppose that a certain volume of hydrogen, which we shall call 1 volume, is caused to combine with the other element, and that the volume of the gaseous compound is measured, the temperature and pressure at which all measurements are made being the same. The data are these ;— 1 volume of hydrogen combines with 1 volume of chlorine to form 1 volume of hydrogen combines with 1 volume of bromine to form Let there be a molecules of hydrogen in the 1 vol. used, then the data translated into the language of the Avogadrean hypothesis read thus ; x molecules of hydrogen combine with x molecules of chlorine and produce 2x molecules of hydrogen chloride. x molecules of hydrogen combine with x molecules of bromine and produce 2x molecules of hydrogen bromide. Now as every molecule of hydrogen chloride is composed of both hydrogen and chlorine, and as every molecule of hydrogen bromide is composed of both hydrogen and bromine, the necessary conclusion-if we grant Avogadro's hypothesis -is that one molecule of hydrogen chloride (or bromide) is composed of half a molecule of hydrogen and half a molecule of chlorine (or bromine); in other words, that each molecule of hydrogen, and each molecule of chlorine and bromine, has separated into at least two parts, and that these parts of molecules have combined to produce the molecules of the compounds formed in the reactions. If we now tabulate the data for the reverse chemical changes, and translate these data into the language of Avogadro's hypothesis, we have the following statements :— volumes of hydrogen chloride produce 1 volume of hydrogen and 1 volume of chlorine. 2x molecules of hydrogen chloride produce x molecules of hydrogen and x molecules of chlorine. volumes of hydrogen bromide produce 1 volume of hydrogen and 1 volume of bromine. 2x molecules of hydrogen bromide produce x molecules of hydrogen and x molecules of bromine. As we concluded from the former data that a single molecule of hydrogen reacting with a single molecule of chlorine (or bromine) produces 2 molecules of hydrogen chloride (or bromide), so now we conclude that 2 molecules of hydrogen chloride (or bromide), when decomposed produce 1 molecule of hydrogen and 1 molecule of chlorine (or bromine). The outcome of Avogadro's conception of the structure of 286 matter is given in the statement already enunciated; equal volumes of gases contain equal numbers of molecules. The application of this generalisation to the interactions between hydrogen and chlorine, and hydrogen and bromine, has led to the conclusion that the molecules of these elementary gases are composed each of at least two parts, and that these parts part company when the gases interact to form hydrogen chloride and bromide, respectively. Since the time of Avogadro the physical conception of the 287 molecule, as a minute portion of matter, has been much advanced. Every attempt to gain a consistent notion of the mechanism of physical changes has led to the recognition of the grained structure of matter. The hypothesis which asserts that a mass of apparently homogeneous matter is really homogeneous, that however small are the parts into which the body is divided each part exhibits all the properties of the body, has failed to explain any large class of physical facts. Physicists have fully adopted the view that a quantity of any kind of matter consists of a vast number of very minute particles in constant motion. These minute portions of matter they call molecules. The molecules of a gas are supposed to be continually moving about, frequently colliding against each other and rebounding again, but yet remaining intact during 288 289 290 these collisions. The physical definition of the molecule of a gas is given in the following words of Clerk Maxweli. "A gaseous molecule is that minute portion of a substance which moves about as a whole, so that its parts, if it has any, do not part company during the motion of agitation of the gas." The theory that every portion of a body we can see or handle is composed of a great number of very minute particles, in constant motion, each of which is possessed of the properties which characterise the body in question, does not assert or deny the infinite divisibility of matter. What this theory asserts, to use the words of Clerk Maxwell, is "that after we have divided a body into a certain finite number of constituent parts called molecules, then any further division of these molecules will deprive them of the properties which give rise to the phenomena observed in the substance." The relations between the motions and the space occupied by a number of molecules which are mutually independent have been investigated by mathematical analysis. The equations arrived at, after making a justifiable assumption as to the dynamical meaning of temperature, express with considerable accuracy the observed relations between the volume, temperature, and pressure, of gases considerably removed from their liquefaction-points; that is to say the equations agree well with the laws of Boyle and Charles. The properties of a system of molecules moving about freely, and acting on each other only when they come into contact, have been investigated mathematically. One of the deductions arrived at is the generalisation which was stated by Avogadro in 1811; 'Equal volumes of gases contain equal numbers of molecules.' This generalisation is thus raised from a merely empirical statement to the rank of a deduction, made by dynamical reasoning, from a simple hypothesis regarding the structure of matter, which is itself justified by many classes of experimentally established facts. The generalisation of Avogadro is of fundamental importance in chemistry. It is essential that the student should understand that this statement rests on physical evidence and dynamical reasoning; and also that he should understand that the statement presupposes the physical definition of the molecule of a gas (s. par. 287). When this generalisation is applied to many chemical changes taking place between gaseous elements, it leads to the necessary conclusion that the molecules of most gaseous elements are composed of parts, and that these do part company when the molecules chemically interact. Hence in chemistry we must recognise two orders of small particles; molecules, and the parts of molecules or atoms. Avogadro's generalisation, or Avogadro's law* as it is 291 usually called, furnishes a means for determining the relative weights of gaseous molecules. For, if the number of molecules in equal volumes of two gases (at the same temperature and pressure) is the same, it follows that the ratio of the densities of the gases is also the ratio of the masses of the two kinds of molecules. Now a specified volume of oxygen is 16 times heavier than an equal volume of hydrogen; therefore a molecule of oxygen weighs 16 times as much as a molecule of hydrogen. Therefore if the weight of the molecule of hydrogen is taken as unity, the molecular weight of oxygen must be 16. But ought the molecular weight of hydrogen to be taken 292 as unity? We have already found that the application of Avogadro's law to the interactions which occur between hydrogen and chlorine, and hydrogen and bromine, requires us to assert that each molecule of hydrogen separates, in these reactions, into at least two parts. A similar examination of other reactions between hydrogen and various gaseous elements confirms this conclusion. A molecule of hydrogen then is composed of at least two parts, or atoms. But we agree to call the atomic weight of hydrogen one, and to make this the standard in terms of which the relative weights of the atoms of other elements are to be stated. Hence the smallest value which we can give to the molecular weight of hydrogen is two. Of course we may assume that when hydrogen and chlorine react, each molecule of either element separates into 4, 6, 8, 10, &c. parts or atoms; we must assert that each separates into at least two parts. Suppose the assumption is made that each molecule separates into 4 atoms; then, as there are twice as many molecules of hydrogen chloride formed as the number of molecules of hydrogen or chlorine taking part in the reaction, it follows that each molecule of hydrogen chloride is composed of 2 atoms of hydrogen and 2 atoms of chlorine. But no chemical reactions of hydrogen chloride are in keeping with this conclusion. When this compound is decomposed with separation of * The student should observe that the term law is used here in a sense different from that in which the same term is applied to the facts of chemical combination: s. note at end of Chap. xvi. |
