Nitrate of Silver. Nitrate of Lead. Basic Nitrate of 0 0 0 0 0 A g-0— —0—Pb-0—Pb--0—N I!) 4!) 4!) 4!) 4:!4 We at once see the connection between the dyad character of lead (—Pb--), and the occurrence of basic salts.
Similarly we know that if we pass a current of chlorine gas into a cold solution of caustic potash, we obtain a mixture of chloride of potassium and hypochlorite of potash 2KHO = KCI But that if, instead of caustic potash, we take slaked lime, we obtain, not a mixture, but a single substance: 012 = Putting the new symbols into a graphic form we have: Cl Cl—K 4- 11-0—H, and11-0—K 01-0—K Cl H-0--, Cl—, Ca = Ca + H—O—H.
C1-0 Where we see why we have a mixture in the first case, and a single substance in the second, the reason being that calcium being a dyad, one atom of it represents two atoms of potassium. Many other examples might be given, but these may suffice as an indication of the reasons which have induced chemists to prefer the atomic weights given in the second column in the table in the article ATOMIC WEIGHTS.
Assuming, then, these atomic weights, let us return to the subject of Chemical Struc ture. This.may be defined in various ways, but most conveniently as the indication by a graphic formula, or something equivalent to it, of all the chemical changes by which the substance can be formed or decomposed. This will best be illustrated by means of a few examples, and we shall select these from among organic compounds, that is, compounds of carbon, because the structure of these compounds has been most fully investigated.
Acetic acid has (on the new system, which will be exclusively used in the remainder of this article) the formula If it is treated with caustic potash, it yields acetate of potash according to the equation ±KHO = Here one atom of hydrogen has been replaced by one atom of potassium, and we find that further treatment with caustic potash does not cause any further replacement of hydro gen by potassium. We may therefore write the formula of acetic acid thus: 13—(Cill 302). and this formula indicates the replaceability of one atom of hydrogen by metal, and explains (as far as such formulae can explain anything) the occurrence of such com pounds as acetate of lead and all the other acetates. The question now remains, what is the structure of the group (0211,02), which is united in acetic acid to hydrogen, and in the acetates to metal? To answer it we must examine some other reactions of acetic acid. When treated with pentachloride of phosphorus, it loses an atom of oxygen, the place of which is taken by two atoms of chlorine—the pentachloride of phosphorus taking the oxygen in exchange for the chlorine; but instead of obtaining a compound we find that the result is expressed by the equation : C211402 4- PC14 = + H01-1- POC1s. We thence conclude that in acetic acid the atom of dyad oxygen removed in the action given above was united to an atom of hydrogen, and to the group and repre sent the change thus: