Or H_{5}C_{2}O.C_{2}H_{3}O, or H_{5}C_{2}.C_{2}H_{3}O_{2}. Now each of these two latter formulæ is a partial formula, each represents a one-sided view; it is justifiable if you use both, but unfair if you use only one. We now come to the question as to the existence or non-existence of two distinct classes of compounds, one in which the atoms are combined directly or indirectly with each other, and the other in which a group of atoms is combined as an integer with some other group of atoms, without any atomic connection by so-called molecular combination. These two modes of combination are essentially distinct. The question is not one of degree. Are there any facts to support this theory that one set of compounds is formed in one way, another in a different way? Take the case of the sulphates: Starting with SO_{3}, we can replace one atom of O by HO_{2}, and obtain SO_{2}(HO)_{2} or H_{2}SO_{4}; replacing a second atom, we get SO(HO)_{4} or H_{4}SO_{5}, glacial sulphuric acid, a perfectly definite body corresponding to a definite class of sulphates, e.g., H_{2}MgSO_{5}, Zn_{2}SO_{5}, etc. By replacing the third atom of O we get S(HO)_{6} or H_{6}SOH_{6}; this corresponds to a class of salts, gypsum, H_{4}CaSO_{6}, etc. These are admitted without dispute to be atomic compounds. Are we to stop here? We may write the above compounds thus: H_{2}SO_{4}, H_{2}SO_{4}H_{2}O, H_{2}SO_{4}2H_{2}O. If we measure the heat evolved in the formation of the two latter compounds, it is, for H_{2}SO_{4}+H_{2}O, 6.272; H_{2}SO_{4}+2H_{2}O, 3.092. But if we now take the compound H_{2}SO_{4}+3H_{2}O we have heat evolved 1.744; so we can have H_{2}SO_{4}4H_{2}O, etc. Where are we to draw the line between atomic and molecular combination, and why? It comes to this: All compounds which you can explain on your views of atomicity are atomic, and all that you cannot thus explain are molecular. Similarly with phosphates, arsenates, etc. In all these compounds it is impossible to lay one's finger on any distinction as regards chemical behavior between the compounds called atomic and those usually called molecular. Two points remain to be mentioned: The first is the relationship between alteration of adicity and two series (ous and ic) of compounds. Tin is usually said to be dyad in stannous compounds and a tetrad in stannic compounds, but in a compound like SnCl_{2}AmCl, is not tin really a tetrad? {Cl {Cl Sn {Cl {NH_{4}