16. Lithium 1817 17. Selenium 1817 18. Bromine 1826 19. Aluminium 1827 20. Thorium 1828 21. Ruthenium 1828–1845 22. Magnesium 1830 23. Lanthanum 1839 24. Terbium. Studied about 1839 (Not yet isolated.) 25. Erbium 1843 26. Neodymium 1843 27. Praseodymium 1843 28. Rubidium 1860 29. Cæsium 1860 30. Thallium 1861 31. Indium 1863 32. Gallium 1875 33. Decipium. (Name given in 1878 to mixture of Samarium and Decipium.) Isolated 1878 34. Ytterbium 1878 35. Thulium. (Name given by Cleve in 1879 to a metal in Gadolinite. Has not yet been isolated, and elementary nature is disputed.) 36. Scandium. Known since 1879 (Not yet isolated.) 37. Germanium 1885 38. Samarium. (A name given to a metal found in Gadolinite. Elementary nature very doubtful.) 39. Holmium. (Not yet isolated.) 40. Argon 1895 41. Helium 1896 42. Metargon 1898 43. Krypton 1898 44. Neon 1898 45. Polonium 1898 46. Coronium 1898 47. Xenon 1898 48. Monium 1898 49. Etherion (?) 1898 50. Gadolinium (?) 1885 51. Radium (?) 1898 The date in each case is that of the discovery. Numbers 49, 50, and 51 are not yet sufficiently well known to justify being considered elements, and are therefore properly followed by an interrogation point. II. PHYSICAL CHEMISTRY. In strictly physical chemistry the relations of electricity and heat to chemical action have been extensively developed during the century. The specific heats of the elements and of most of their compounds have been carefully determined, and thermo and physical chemistry under the leadership of such master minds as Berthollet, Thompson, Van’t Hoff, and Ostwald have been brought to the highest degree of perfection. The chemist now does not consider that he knows any body until he knows thoroughly its relations to heat and to electricity. The action of light must also be included, but this subject will be more thoroughly discussed under graphic chemistry.