In 1807 Dalton published a work entitled “New System of Chemical Philosophy,” in which was announced for the first time the law of the definite proportions of bodies forming a definite union. The atomic theory of matter was also developed by Dalton, who gave it a definite form and expression. Chemists now began to consider the elements as definite indestructible particles of matter, forming unions among themselves and with different kinds of atoms to form molecules, which were considered as the units of substances. As a result of this supposition, the development of the principle of the relative weight with which bodies combine was the logical consequence.

Now for the first time the elements began to assume not only names and descriptions of properties but also numbers, showing the relative weight of their atoms or final conditions of existence. It was only necessary, therefore, to assume the standard of comparison for any one element, in order to determine the relative weights with which it combined with others. Thus the system of atomic weights was developed.

As a result of the law of chemical action, that most elementary bodies exist in a condition where two atoms are joined together to form a molecule, it follows, that in most instances the molecular weights of the elements are double their atomic weight. There are, however, many notable exceptions to this rule.

The supposition of the existence of atoms was followed soon by another theoretical proposition, advanced by Prout (1815). Assuming that the atomic weight of hydrogen was one, Prout’s hypothesis asserted that the atomic weights of all other elementary bodies were multiples of that of hydrogen. The most rigid investigations of recent years have shown that Prout’s hypothesis is untenable; but the remarkable fact still remains, that in a great many cases the atomic weights of the elements are almost whole numbers, or differ from whole numbers by almost a half unit.

The determination of the atomic weights of the various elements during the past one hundred years has been worked on by hundreds of chemists whose names it would be impracticable to mention. The most important of them are Berzelius, Cooke, Cleve, Delafontaine, Dumas, Hermann, Marchand, Marignac (1817), Morley, Noyes, Pelouse (1807–1867), Richards, Schneider, Stas (1813–1891), and Thompson. Of all these workers Stas, a Belgian chemist, is perhaps the most renowned. Among those mentioned, Cooke, Morley, Noyes, Delafontaine, and Richards are citizens of the United States.

From the less than thirty elements which were known at the beginning of the century, there are known to-day seventy-two with certainty, and perhaps one or two more whose identity has not yet been fully established. The chemists who have become most renowned by the discovery of elementary bodies are: Cavendish, Scheele, Berzelius, Wöhler (1800–1882), Davy, Gay-Lussac, Priestly, Bunsen (b. 1811), Crookes (b. 1832), and Ramsay.

The following elements, twenty-eight in number, were known before 1800:

ELEMENTS KNOWN BEFORE 1800.
 1. CopperKnown to Ancients.
 2. Gold” ” ”
 3. Iron” ” ”
 4. Lead” ” ”
 5. Silver” ” ”
 6. Tin” ” ”
 7. Carbon” ” ”
(But three forms not identified until 1786–1800.)
 8. MercuryKnown to Ancients.
 9. AntimonyFifteenth Century.
10. Bismuth” ”
11. Zinc” ”
12. Phosphorus1669
13. Arsenic (Isolated)1697
” (Studied)1733
14. Cobalt1733
15. Platinum1735–1748
16. Nickel1751
17. Hydrogen1766
18. Nitrogen1772
19. Oxygen1774
20. Manganese (Studied in compounds, isolated at unknown date)1774
21. Barium1774
22. Tungsten1781–1785
23. Molybdenum1782
24. Tellurium1782–1798
25. Strontium1790
26. Yttrium1794
27. Chromium1797
28. Beryllium1798

Four additional elements were known to exist before that date, but they had not been isolated and identified. These are:—

ELEMENTS KNOWN BUT NOT ISOLATED OR EXAMINED BEFORE 1800.
Chlorine{Compound known1774
{Isolated and studied1810
Titanium{Known in compounds1791
{Isolated1824
Uranium{Known in compounds1789
{Isolated1824
Zirconium{Known in compounds1789
{Isolated1824