读过普通化学的人,都知道有元素的週期表(Periodic table)。週期表把错综复杂的元素,排列成为有系统的表式,很明晰地加以分类,这对于近代化学的进展,无疑的有很重要的贡献。原来到十九世纪前期为止,化学家虽然可藉各种鹼金属和分光术的帮助,来发见若干未知之元素,但是每一种发见,都可说出于偶然。所以在有週期表以前,化学家对于未知之元素无法预知,至于这种未知元素之物理的或化学的性质,自然更无法推测的。因为这个缘故,有许多化学家注意到元素分类的研究,使化学成为一种系统的科学。这里应当提起的,有英国的J.A.R.Newlards,德国的Lot People who read normal chemistry know that there are periodic table of elements. The periodic table organizes intricate elements into systematic expressions that are clearly categorized. This undoubtedly has an important contribution to the progress of modern chemistry. It turned out that by the early nineteenth century, though chemists could see some unknown elements with the help of various kinds of alkali metals and spectrophotometers, each of them could be said by accident. So before the periodic table, chemists are unpredictable of unknown elements, and the physical or chemical properties of this unknown element are, of course, more unpredictable. For this reason, many chemists have noticed the study of elemental classification, making chemistry a systematic science. Mention should be made here of J.A.R. Newlards in the United Kingdom, Lot in Germany