Occasionally the development of a new technique would lead to the "easy" discovery of a whole group of new elements. Thus Davy, starting in 1807, applied the method of electrolysis, using a development of Volta's pile as a source of current; in a short time he discovered aluminum, barium, boron, calcium, magnesium, potassium, sodium, and strontium.
The invention of the spectroscope by Bunsen and Kirchhoff in 1859 provided a new tool which could establish the purity of substances already known and lead to the discovery of others. Thus, helium was discovered in the sun's spectrum by Jansen and isolated from uranite by Ramsay in 1895.
The discovery of radioactivity by Becquerel in 1896 (touched off by Roentgen's discovery of x rays the year before) gave an even more sensitive method of detecting the presence or absence of certain kinds of matter. It is well known that Pierre and Marie Curie used this new-found radioactivity to identify the new elements polonium and radium. Compounds of these new elements were obtained by patient fractional recrystallization of their salts.
The "explanation" of radioactivity led to the discovery of isotopes by Rutherford and Soddy in 1914, and with this discovery a revision of our idea of elements became necessary. Since Boyle, it had been assumed that all atoms of the individual elements were identical and unlike any others, and could not be changed into anything simpler. Now it became evident that the atoms of radioactive elements were constantly changing into other elements, thereby releasing very large amounts of energy, and that many different forms of the same element (lead was the first studied) were possible. We now think of an element as a form of matter in which all atoms have the same nuclear charge.
The human mind has always sought order and simplification of the external world; in chemistry the fruitful classifications were Dobereiner's Triads (1829), Newland's law of octaves (1865), and Mendeleev's periodic law (1869). The chart expressing this periodic law seemed to indicate the maximum extent of the elements and gave good hints "where to look for" and "the probable properties of" the remaining ones (see Fig. 2).
By 1925, all but four of the slots in the 92-place file had been filled. The vacancies were at 43, 61, 85, and 87.