Page:The American Cyclopædia (1879) Volume XI.djvu/735

 MOLECULE 717 assumes that the atoms are not only invariable quantities, but definite bodies, and that the molecules are congeries of atoms, except, of course, in those cases where the molecules consist of a single atom. The atom is the unit of the chemist in the same sense that the molecule is the unit of the physicist ; and as the molecules are the limits which the sub- division of matter reaches in any physical pro- cess, so what we now regard as atoms are the limits of the subdivision in any known chem- ical process. The word atom, by which we designate these units of chemistry and which we have inherited from the past, is in some respects an unfortunate term, because its ety- mology suggests conceptions which are no longer associated with the small masses it des- ignates. Although our chemical atoms have never as yet been divided, we do not regard them as necessarily indivisible. The atom of oxygen for example, weighing 16 microcriths, is simply the smallest quantity of this elemen- tary substance known to exist in a molecule of any of its compounds. It is by no means impossible that a compound of oxygen may be discovered whose molecule contains only 8 microcriths of the element; or in other words, that what we now call an oxygen atom may be divided, just as it is possible that the elementary substance may be decomposed ; for the chemical atom, like the chemical element, is such only provisionally. The distinction between atom and molecule is preserved in chemistry by the system of chemical symbols. The Latin initials which are used as the sym- bols of the chemical elements represent in every case one atom of the elementary sub- stance, and the groups of these letters which are used as the symbols of chemical compounds stand in each case for one molecule of the compound. Thus O stands for one atom or 16 microcriths of oxygen, C for one atom or 12 microcriths of carbon, and H for one atom or one microcrith of hydrogen ; H a O for one molecule of water, C 2 H 6 O for one molecule of alcohol, CalUOa for one molecule of acetic acid. The figures below the symbol indicate the number of atoms of the corresponding ele- ment in the molecule, and the weight of the molecule is obviously the sum of the weights of the atoms which the symbols represent. For example, the weights of the molecules of water, alcohol, and acetic acid are 18, 46, and 60 microcriths respectively. Several molecules of a compound may be indicated by placing figures before the group of symbols like an algebraic coefficient ; thus 3H 2 O stands for three molecules of water, and 4C 2 H 4 O 2 for four molecules of acetic acid. In order to determine the symbol of a molecule, we must know in the first place the molecular weight, and in the second place the percentage of com- position. The molecular weight, as we have seen, is found approximately from the gas or vapor density, and corrected by the combining proportions of the substance. The percentage ELEMENTS. Analytii of butyric acid. Composition of one molecule. Atomic weight. Number of atoms in on molecule. Carbon.... Hydrogen.. Oxygen. . . 64-51 9-26 86-23 47-97 m.c.= 8-15 " = 81-88 " = 12 x Ix ICx 4 8 2 100-00 88 " composition is determined by a quantitative chemical analysis. As an example, suppose we wished to determine the symbol of butyric acid. The specific gravity of the vapor of this volatile compound was determined by Cahours and found to be 44*3, hydrogen gas being unity. Hence the molecular weight would be 88*6 microcriths, but the combining propor- tions of the acid deduced from analyses of its salts prove that the more accurate value is 88 m. c. An analysis of the acid published by Grunzweig gives the following percentage composition, and from this we easily calculate the amount of each element in one molecule : m. c. or Evidently in the molecule there are 4 atoms of carbon, 8 atoms of hydrogen, and 2 atoms of oxgen, and the symbol is therefore C4H 8 2 . Thus we can fix the symbols of all volatile bodies. If the substance is non-volatile, we rely primarily on the combining proportion to fix the molecular weight, and, as has been said, there is frequently danger of assigning mul- tiple values. But as a general rule the chem- ical relations of the substance enable us to de- termine which of the possible multiples should be taken ; and if these leave us in doubt, we adopt provisionally as the symbol of the mo- lecule that multiple which has the smallest number of whole atoms, and wait for the pro- gress of science to correct any error. But even after weighing the molecules and deter- mining the number and kind of atoms of which they consist, chemistry has gone for- ward still further in developing the molecular theory, and has discovered a large and impor- tant class of phenomena, which it refers to dif- ferences of arrangement in the grouping of the atoms of the molecules. Thus, there is a sub- stance called acetic ether which has the same molecular weight and the same percentage of composition as butyric acid. Its molecules therefore contain the same number of the same kind of atoms as the other. Yet the qualities of the two substances are utterly different, the acid having the disgusting smell of rancid but- ter and the ether the pleasant odor of apples, and chemistry attempts to explain this differ- ence by showing that the atoms are arranged differently in the two classes of molecules, as the following diagrams indicate : O H H H II I II H C C C C H J U Butyric acid. H H OH i_i_(i_o_c c n U A Acetic ether.