It is vain to hope for a knowledge of the _absolute_ weight of the ultimate atoms of matter, and nothing seems to be more beyond the power of man than to determine even their relative weights; yet the definite proportions in which they combine have enabled him to do so. Thus, an atom of oxygen unites with an atom of hydrogen to form water; but as every drop of water, however small, contains eight parts by weight of oxygen, and one part by weight of hydrogen, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. Now, since hydrogen gas is the lightest body known, its atom has been assumed as the unit of comparison. Hence, if the unit of hydrogen be represented by 1, that of oxygen may be represented by 8. Again, carbonic acid gas contains six parts by weight of carbon, and eight parts by weight of oxygen, and as an atom of oxygen is eight times heavier than an atom of hydrogen, therefore an atom of carbon is six times heavier than an atom of hydrogen, and consequently may be represented by 6. In this manner the relative weights of many substances have been determined. But the property of isomorphism also affords the means of ascertaining the atomic weights of certain substances with unerring certainty. It is exactly the contrary of dimorphism, for in the latter substances are chemically the same under different forms; whereas isomorphic bodies are chemically different under the same form. Now the peroxide of manganese contains one atom of oxygen for one atom of metal; but in 100 parts of the protoxide there are 21·94 parts of oxygen and 78·06 of manganese. Comparing these numbers with 8 the atomic weight of oxygen, the result is 28 the weight of an atom of manganese. The same number is obtained from two other isomorphic compounds of oxygen and manganese, which proves the accuracy of this result. The atomic weights of many bodies have been determined, of which the following are the most important. _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.