AVOGADRO'S RULE. one of the funda mental principles of chemistry. It is usually formulated as follows: Under the same eondi tions of temperature and pressure equal volumes of gases and vapors contain equal numbers of molecules. It may appear strange, at first sight, that an idea of this nature should be incorpo rated in the very foundation of an exact science like chemistry. For the very conception of mat ter as made up of molecules is nothing but an hypothesis: and even if this conception is correct, the number of particles in a given mass of mat ter could of course never be actually counted. The following considerations, however, will show at once the real significance of Avogadro's rule. An immediate consequence of the rule is, that comparing equal volumes of substances in the gaseous state, as to weight, composition, etc., is the same as comparing single molecules. Fol lowing the rule, chemists have therefore adopted as the standard quantity for investigation, not a unit of weight, but a unit of volume, and have for many years now been comparing, not equal weights of substances, but equal volmnes. Nov, this practice has led to the acquirement of a stupendous amount of purely empirical informa tion that forms a solid part of the science of chemistry, the original rule merely explaining achy comparing equal volumes should lead to so many excellent results. If, therefore, the atomic hypothesis should sometime be discarded as no longer necessary, the hypothetical form of Avo gadro's rule would of course go with it but its corollary, the expediency of comparing equal volumes of substances, would still remain a great guiding rule in the science of chemistry.
The rule was proposed as an addi tbm to the atomic theory, the purpose being to account for a remarkable general fact then re cently discovered by Gay Lussac; viz., that when two gases react with each other chemically, their reacting volumes bear to each other a ratio that can be expressed by small integral numbers. Thus, when hydrogen and chlorine unite to form hydro chloric acid, the volumes of the reacting gases are equal, i.e. their ratio is 1 ± 1. Similarly, equal volumes of hydrochloric acid gas and ammonia combine to form sal-ammoniac; i.e. the ratio is again 1 1. In the formation of water from hydrogen and oxygen, these two gases unite in the simple ratio of 2 1. Gay Lussac's law holds equally good in the case of gaseous products of chemical decomposition and with regard to the ratios of the volume of a gaseous compound to the volumes of its gaseous chemical components.
Thus, 3 volumes of hydrogen and 1 volume of nitrogen are produced by the decomposition of 2 volumes of ammonia. This law, examined from the standpoint of the atomic hypothesis, indi cates that some very simple relation must exist between the numbers of particles contained in the reacting volumes of gases. For according to that hypothesis all chemical reactions of sub stances take place really between their particles. The simplest relation that suggests itself to the mind is expressed by Avogadro's rule; viz., that equal volumes of gases contain equal numbers of molecules. If, then, the reacting volumes of hydrogen and chlorine are equal, it is because every single molecule of chlorine reacts with one single molecule of hydrogen. Although it thus formed a plausible explanation of Gay Lussac's law, the idea, when first advanced by Avogadro in 1811 and resuggested by Ampere in 1814, was not accepted by the scientific world. The scanty stock of experimental knowledge of the time did not warrant the incorporation in science of a general theoretical principle of this nature.
The practical importance of the rule is mainly in the fact that it permits of ascertaining the relative weights of molecules (`molecular weights'), these weights being represented, ac cording to the rule, by the relative weights of equal volumes of substance in the gaseous state. The substance with which all other substances are usually compared is hydrogen, to which chemists assign the molecular weight 2, since its molecule is supposed to consist of two atoms of unit atomic weight (the unit is of course arbitrary and is chosen merely for convenience sake). Comparing equal volumes of hydrogen and oxygen, the latter is found to weigh 16 times as munch as the former. The molecular weight of oxygen is, therefore, in accordance with Avo gadro's Rule, said to be 32. In a similar man ner the molecular weight of any other substance in the gaseous state may be ascertained by mul tiplying the density (i.e. its relative weight with respect to hydrogen) by 2. Thus, the density of ammonia gas being about 8.5, its molecular weight is 17; the density of hydrochloric acid gas being about 18.25, its molecular weight is 36.5, etc.