Robert Boyle in his "Sceptical Chymist" (1661) first defined the word element in the sense which it retained until the discovery of radioactivity (1896), namely, a form of matter that could not be split into simpler forms.
The first discovery of a true element in historical time was that of phosphorus by Dr. Brand of Hamburg, in 1669. Brand kept his process secret, but, as in modern times, knowledge of the element's existence was sufficient to let others, like Kunkel and Boyle in England, succeed independently in isolating it shortly afterward.
As in our atomic age, a delicate balance was made between the "light-giving" (desirable) and "heat-giving" (feared) powers of a discovery. An early experimenter was at first "delighted with the white, waxy substance that glowed so charmingly in the dark of his laboratory," but later wrote, "I am not making it any more for much harm may come of it."
Robert Boyle wrote in 1680 of phosphorus, "It shone so briskly and lookt so oddly that the sight was extreamly pleasing, having in it a mixture of strangeness, beauty and frightfulness."
These words describe almost exactly the impressions of eye witnesses of the first atom bomb test at Alamagordo, New Mexico, July 16, 1945.
For the next two and three-quarters centuries the chemists had much fun and some fame discovering new elements. Frequently there was a long interval between discovery and recognition. Thus Scheele made chlorine in 1774 by the action of "black manganese" (manganese dioxide) on concentrated muriatic acid (hydrochloric acid), but it was not recognized as an element till the work of Davy in 1810.
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.