_Atomic Weights, an Atom of Hydrogen being the Unit._
Hydrogen 1 Carbon 6 Oxygen 8 Nitrogen 14 Sulphur } 16 Phosphorus } Sodium 23 Iron } Nickel } 28 Manganese } Bromine 80 Copper 32 Zinc 32·5 Chlorine 35·5 Potassium 39 Rubidium 86 Cæsium 133 Iodine 127 Thallium 204
In the determination of atomic weights a few cases have occurred of fractional numbers, and although it cannot yet be affirmed that no such cases exist, yet it seems to be established by the new and more perfect analyses of MM. Dumas, Isidore, Williamson, and others, that the atomic weights of substances compared with an atom of hydrogen are in whole numbers.
This law leads to very important results. For example, the equivalent weights of the chemical elements of bodies derived from their specific gravities are either identical with, or simple multiples or sub-multiples of, their relative weights. Thus the specific gravity of hydrogen is 0·0693, and that of oxygen is 1·111; hence taking hydrogen as the unit of comparison, it is easy to see that 0·0693 : 1·111 :: 1 : 16, the simple multiple of 8, the relative atomic weight of oxygen. In fact since each substance has its own specific gravity or weight, that weight must depend upon the weight of its atoms, so that the weights of equal bulks of different substances are proportional to the weights of their atoms, and thus a relation is established between the atomic weights and specific gravities of bodies, so that one being given the other may be found.
Atoms like their substances have many different capacities for heat and electricity. It was proved by MM. Petit and Dulong, that specific heat, or the quantity of heat required to raise a simple substance to a given temperature, is inversely as the weight of its atoms, so that the specific heat or repulsive force of simple substances multiplied by their atomic weights is a constant quantity. Such is the condition requisite for the equilibrium or equality of force; or the law may be thus expressed: A given quantity of heat will raise to the same number of degrees a portion of every simple substance represented by its atomic weight. For instance, the atomic weight of sulphur is 16, that of zinc 32·5; hence it requires twice as much heat to raise a pound of sulphur ten degrees as it does a pound of zinc. It has also been proved that the atoms of compound bodies of analogous composition are endowed with the same capacity for heat, so that there is a perfect correspondence between the weight of atoms and their specific heat. The numbers representing the atomic weights derived from the specific heat of bodies are connected with their equivalent atomic weights by the simple ratios of equality, multiples or sub-multiples.