This is merely another way of saying that if you know the atomic weight of an element, you also know its properties, since these are fixed or invariable. Mendeleeff arranged the elements in various “Groups,” according to their atomic weights, and it was found that the properties of the elements periodically recur as the weights of the atoms rise. There were certain empty spaces in Mendeleeff’s Table, waiting for new elements which should fit into these empty spaces, if discovered. At the time they had not been discovered; but several of them have been since, and it is a remarkable fact that they invariably fit into his table exhibiting all the properties which they should theoretically exhibit, and might have been predicted to, years before. This is one of the surest confirmations of the accuracy of Mendeleeff’s general Law, and is one of the finest generalizations ever made in science. The conclusion which we may draw from this Law is that there is a definite _relationship_ between the chemical elements. How or why this relationship existed was not known at the time, and only became clear half a century later, when the newer discoveries concerning the ultimate constitution of matter rendered this clear. EARLIER DISCOVERIES Mendeleeff’s Law could not have been formulated had not an immense amount of research work preceded it, and a number of new elements been discovered. Such was, however, the case. Immediately following the great work of Lavoisier, a host of brilliant chemists appeared, and rapid and important advances were made in consequence. Cadmium was discovered by Stromeyer in 1817; lithium in the same year by Arfvedson. Silicon was isolated in 1810 by Berzelius. In 1827, Wohler isolated aluminum; and the same scientist also isolated beryllium the following year. Bromine was discovered by Balard in 1826; iodine, in 1811, by Courtois. Tellurium had been discovered by Muller von Reichenstein in 1782; Berzelius discovered an element closely analogous to it--selenium--in 1817. Elements continued to be added to the list--and then no more! Had every element been discovered? Some were inclined to think so. With the discovery of Radium, by the Curies, however, another whole list of elements was brought to light--all of which have been added to the Table of the Periodic Law.