Now, equal volumes of ammonia gas and hydro chloric acid combine to form ammonium chloride (sal-ammoniac, NILO]) and nothing else. Both the chlorine of hydrochloric acid and the nitro gen of ammonia represent the smallest relative weights of those elements found in any compound; or, in the language of the atomic theory, hydro chloric acid contains only one atom of chlorine, and ammonia only one atom of nitrogen. Hence, one entire molecule of hydrochloric acid (molecu lar weight = 36.5) and one entire molecule of ammonia (molecular weight = 17) must be com bined in one molecule of ammonium chloride; for otherwise the latter compound could not, as it does, contain both chlorine and nitrogen. Con sequently, the molecular weight of ammonium chloride could not possibly be less than 17 + 36.5 = 53.5, and hence its density could not, according to Avogadro's rule, be less than 53.5 2 = 26.75. When, however, ammonium chloride is heated and its vapor studied experi mentally, the density is found to be only about one-half as great. This fact was for a long time regarded as proving that Avogadro's rule is incapable of general application, although in most other cases the molecular weights obtained with the aid of the rule were found to agree thoroughly with the chemical behavior of the compounds. But even in the case of ammonium chloride, the exception was finally shown to be only apparent. Pebal and Than, namely, demon strated that when ammonium chloride is evapo rated, it breaks up into its constituents, am monia gas and hydrochloric acid, the dissocia tion naturally causing an increase of volume, and hence a decrease of density. As long as the products remain mixed in a vessel, or are al lowed to escape together, it is impossible to prove directly that the vapor is a mixture of ammonia and acid, the reaction with litmus paper being of course neutral. But Pebal and Than proceeded as follows: They divided a glass tube into two parts by a porous partition and heated a lump of ammonium chloride in one part of the tube; the ammonia gas produced, diffusing through the partition more rapidly than the hydrochloric acid, soon filled the other part of the tube and showed the ordinary alkaline re action with red litmus paper (i.e. turned it blue). The case of ammonium chloride thus changed from an exception into an additional indication of the correctness of Avogadro's rule.
The principal arguments in favor of the rule may he briefly summed up as follows: (1) Avogadro's rule furnishes the chief method of ascertaining the atomic weights of the chemi cal elements, and there are very strong reasons for believing that the atomic weights thus ob tained represent the true numbers characteriz ing the elements. See PERIODIC LAw.
(2) The modern theories of the chemical con stitution of compounds could not have come into existence and could not be applied in individual eases, if the molecular and atomic weights to which the rule leads were unknown. Avogadro's rule is at the very foundation of those theories; and hence, conversely, the correctness of the theories, as indicated by the numerous triumphs achieved by them for both pure and applied chemistry. is to be considered as strong proof in favor of the fundamental rule.
(3) The strongest argument in favor of Avo gadro's rule in its hypothetical form, lies in the fact that it follows, by mathematical deduction, from the kinetic theory of gases. (See GASES, PROPERTIES OF.) The correctness of this theory, on the other hand, is strongly indicated by the fact that it also leads to those laws of gases which have long been established experimen tally.
Avogadro's rule can evidently hold good only in those eases in which the gases or vapors in question behave very nearly in accordance with the laws characteristic of matter in the per fectly gaseous state; i.e. if at constant tempera ture the volume varies inversely as the pressure, and if under constant pressure the volume varies as the absolute temperature. For if, under equal pressures and temperatures different gases did not always occupy equal volumes, then equal volumes could not always contain equal numbers of molecules.
The rule is sometimes spoken of as the law of Avogadro and Gerhardt, the latter chemist hav ing done much toward establishing it on a firm scientific basis. Gerhardt's pupil. Cannizzaro, continued his researches, and finally succeeded in demonstrating'to the scientific world its great importance. In BSC Van't Hoff showed the rule to be applicable not only to gases and vapors, but also to substances in solution, and as a con sequence, the rule is also often referred to, espe cially by German authors, as the rule of Avo gadro-Van't Hoff. Consult: Avogadro's original memoir on the molecular theory, German trans., Osttrald's Klassiker der exakten Wissensehaften, No. St and Cannizzaro, "Abriss eines Lehrganges der theoretisehen Chemie," Ostira Klassiker, No. 30. See also ATOMIC WEIGHTS; MOLECULES -MOLECULAR WEIGHTS ; SOLUTION: CHEMISTRY.