So again, in order to form the Iodide of Silver, the proportions in which the two salts should be mixed is thus shown. The equivalent of Iodide of Potassium is 166, and that of Nitrate of Silver is 170. These numbers so nearly correspond, that it is common to direct that equal weights of the two salts should be taken. One more illustration will suffice. Supposing it be required to form 20 grains of Iodide of Silver--how much Iodide of Potassium and Nitrate of Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium, will yield an equivalent, or 234 parts, of Iodide of Silver; therefore as 234 : 166 :: 20 : 14·2. Hence, if 14·2 grains of the Iodide of Potassium be dissolved in water, and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver added, the yellow precipitate, when washed and dried, will weigh precisely 20 grains. ON THE ATOMIC THEORY. The atomic theory, originally proposed by Dalton, so much facilitates the comprehension of chemical reactions generally, that it may be useful to give a short sketch of it. It is supposed that all matter is made up of an infinite number of minute atoms, which are elementary, and do not admit of further division. Each of these atoms possesses an actual weight, although inappreciable by our present methods of investigation. Simple atoms, by uniting with each other, form _compound atoms_; and when these compounds are broken up, the elementary constituent atoms are not destroyed, but separate from each other, in possession of all their original properties. In representing the simple atomic structure of bodies, _circles_ may be used, as in the following diagram. [Illustration: Fig. 1. Fig. 2. Fig. 3.] Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of Sulphur united intimately with three of Oxygen; fig. 2 is an atom of Peroxide of Nitrogen, NO{4}; and fig. 3, an atom of Nitric Acid, composed of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO{5}. _The term "atomic weight" substituted for equivalent proportion._--If we suppose that the simple atoms of different kinds of matter _differ in weight_, and that this difference is expressed by their equivalent numbers, the whole laws of combination follow by the simplest reasoning. It is easy to understand that an atom of one element, or compound, would displace, or be substituted for, a single atom of another; therefore, taking as the illustration the decomposition of Iodide of Potassium by Chlorine,--the weight of the latter element required to liberate 126 grains of Iodine is 36 grains, because _the weights of the atoms of those two elementary bodies are as 36 to 126_. So again, in the reaction between Chloride of Sodium and Nitrate of Silver, a compound atom of the former, represented by the weight 60, reacts upon a compound atom of the latter, which equals 170.