+ − + − + −
Ag.Aq + NO3.Aq + Na.Aq + Cl.Aq = AgCl + Na.Aq + NO3.Aq.

The compound, silver chloride, being insoluble in water, is formed by the union of the ions Ag and Cl, and their consequent discharge, forming an electrically neutral compound; while the sodium ions, charged positively together with the NO3 ions, negatively charged, remain in solution.

One more application of the principle may be given. Many observers—Andrews, Favre, and Silbermann, but especially Julius Thomsen, of Copenhagen, and M. Berthelot, of Paris—have devoted much labor and time to the measurement of the heat evolved during chemical reactions. Now, while very different amounts of heat are evolved when chlorine, bromine, or iodine combine respectively with sodium or potassium, the number of heat units evolved on neutralizing sodium or potassium hydroxide with hydrochloric, hydrobromic, hydriodic, or nitric acids is always about 13,500. How can this fact be explained? It finds its explanation as follows: These acids and bases are ionized in solution as shown in the equation:

+ − + − + −
H.Aq + Cl.Aq. + Na.Aq + OH.Aq = H.OH + Na.Aq + Cl.Aq.

Water is the only compound formed; and it is produced by the union of the hydrogen-ion originally belonging to the acid, and the OH or hydroxyl-ion originally belonging to the base. No further change has occurred; hence the uniform evolution of heat by the interaction of equivalent quantities of these acids and bases.

It now remains to give a short account of the greatest generalization which has as yet been made in chemistry. It has been termed the “Periodic Arrangement of the Elements.”

In 1864 Newlands, of London, and Lothar Meyer, late of Tübingen, found that by arranging the elements in the order of their atomic weights certain regularities were to be observed between each element, and in general the eighth in succession from it, in the order of their numerical value. Such similar elements formed groups or quantities; while the elements separating them belong to a period, hence the name “periodic arrangement.” Commencing with lithium, a light, lustrous metal found in silicate in certain minerals, we have the following series:

LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
79.2111214161920
SodiumMagnesiumAluminumSiliconPhosphorusSulphurChlorineArgon
2324.32728313435.540

and so on. It is unnecessary to point out in detail the resemblances between the elements which stand in the vertical columns; but it may be stated that the resemblance extends also to the formulas and properties of their compounds. Thus the chlorides of lithium and sodium are each white soluble salts, of the formulas LiCl and NaCl; oxides of magnesium and of beryllium are both insoluble white earthy powders, MgO and BeO (GeO), and so on. Newlands, in his preliminary sketch, termed this order the “Law of Octaves,” and predicted the existence of certain undiscovered elements which should occupy unfilled positions in the table. Mendeléef, professor at St. Petersburg, in 1869 amplified and extended these relations; and he and Meyer pointed out that the volume occupied by equal numbers of atoms of such elements underwent a periodic variation when the elements are classified as above. The prediction of undiscovered elements was made by Mendeléef in a more assured manner; and in several cases they have been realized. Thus what Mendeléef called “ekaboron” has since been discovered by Lecoq de Boisbandron and named, patriotically, “gallium”; Mendeléef’s “eka-silicon” is now known as “germanium,” discovered by Winkler; and “eka-aluminum” is now Cléve’s “scandium.” Moreover, the atomic weights of cæsium, beryllium, molybdenium, and mercury have been altered so that they fit the periodic table; and further research has justified the alteration.

The valency of these elements increases from right to left, as will be seen by inspection of the following series: