474 CHEMISTRY [FOKMULJ?. supposed that in this compound only two of the oxygen atoms are wholly associated with the sulphur atom, each of the remaining oxygen atoms being united by one of its affinities to the sulphur atom, and by the remaining affinity to an atom of hydrogen; thus O II H S-0 H II O .Again, the reactions of acetic acid, C 2 H 4 2 , show that the four atoms of hydrogen which it contains have not all the same function, and also that the two atoms of oxygen have different functions ; the graphic formula which we are led to assign to acetic acid, viz., H H C H 0=0 H serves in a measure to express this, three of the atoms of hydrogen being represented as associated with one of the atoms of carbon, whilst the fourth atom is associated with an atom of oxygen which is united by a single affinity to the second atom of carbon, to which, however, the second atom of oxygen is united by both of its affinities. It is not to be supposed that there are any actual lands of union be tween the atoms ; graphic formulas such as these merely express the hypothesis that certain of the atoms in a com pound come directly within the sphere of attraction of certain other atoms, and only indirectly within the sphere of attraction of others, an hypothesis to which chemists are led by observing that it is often possible to separate a group of elements from a compound, and to displace it by other elements or groups of elements. Rational formulas of a much simpler descriptiorithan these graphic formulae are generally employed. For instance, sulphuric acid is usually represented by the formula SOo(OH) 2 , which indicates that it may be regarded as a com pound of the group SO 2 with twice the group OH . Each of these OH groups is equivalent in combining or displac ing power to a monad element, since it consists of an atom of dyad oxygen associated with a single atom of monad hydrogen, so that in this case the S0 2 group is equivalent to an atom of a dyad element. This formula for sulphuric acid, however, merely represents such facts as that it is possible to displace an atom of hydrogen and an atom of oxygen in sulphuric acid by a single atom of chlorine, thus forming the compound S0 3 HC1 ; and that by the action of water on the compound S0 2 C1 2 twice the group OH, or water minus an atom of hydrogen, is introduced in place of the two monad atoms of chlorine S0 2 C1 2 + 2HOH = S0 2 (OH) 2 + 2HC1 . Water. Sulphuric acid. Constitutional formulae like these, in fact, are nothing more than symbolic expressions of the character of the com pounds which they represent, the arrangement of symbols in a certain definite manner being understood to convey certain information with regard to the compounds repre sented. Groups of two or more atoms like S0 2 and OH , which are capable of playing the part of elementary atoms (that is to say, which can be transferred from compound to com pound), are termed compound radicles, the elementary atoms being simple radicles. Thus, the atom of hydrogen is a monad simple radicle, the atom of oxygen a dyad pimple radicle, whilst the group OH is a monad compound radicle. It is often convenient to regard compounds as formed upon certain types ; alcohol, for example, may be said to be a compound formed upon the water type, that is to say, a compound formed from water by displacing one of the atoms of hydrogen by the group of elements C 2 H 5 , thus
it
C 2 H 5 , H Alcohol. Chemical Action. Chemical change or chemical action may be said to take place whenever changes occur which involve an alteration in the composition of molecules, and may be the result of the action of agents such as heat, electricity, or light, or of two or more elements or compounds upon each other. Three kinds of changes are to be distinguished, viz., changes which involve combination, changes which in volve decomposition or separation, and changes which in volve at the same time both decomposition and combination. Changes of the first and second kind, according to our present views of the constitution of molecules, are probably of very rare occurrence ; in fact, chemical action appears almost always to involve the occurrence of both these kinds of change, for, as already pointed out, we must assume that the molecules of hydrogen, oxygen, and several other elements are diatomic, or that they consist of two atoms. Indeed, it appears probable that with few exceptions the elements are all compounds of similar atoms united together by one or more units of affinity, ac cording to their valencies. If this be the case, however, it is evident that there is no real distinction between the reactions which take place when two elements combine together and when an element in a compound is displaced by another. The combination, as it is ordinarily termed, of chlorine with hydrogen, and the displacement of iodine in potassium iodide by the action of chlorine, may be cited as examples ; if these reactions are represented, as such re actions very commonly are, by equations which merely express the relative weights of the bodies which enter into reaction, and of the products, thus II + Cl = HC1 Hydrogen. Chlorine. Hydrochloric acid. KI + Cl KC1 + I Potassium iodide. Chlorine. Potassium chloride. Iodine. they appear to differ in character ; but if they are correctly represented by molecular equations, or equations which express the relative .number of molecules which enter into reaction and which result from the reaction, it will be ob vious that the character of the reaction is substantially the same in both cases, and that both are instances of the occurrence of what is ordinarily termed double decom position H 2 + C1 2 = 2HC1 Hydrogen. Chlorine. Hydrochloric acid. 2KI + Potassic iodide. C1 2 = 2KC1 Chlorine. Potassic chloride. Iodine. For chemical action to take place between two bodies it is necessary that they should be in contact, and, therefore, generally speaking, that one of them should be in the state of liquid or gas. In all cases of chemical change energy in the form of heat is either developed or absorbed, and the amount of heat developed or absorbed in a given reaction is as defi nite as are the weights of the substance engaged in the reaction. Thus, in the production of hydrochloric acid from hydrogen and chlorine 22,000 units of heat 1 are 1 A unit of heat is the quantity of heat necessary to raise the tem
perature of 1 gramme of water 1C., and whenever in this article it is
