The Table of Elements Arranged in a Dream
By Mendeleev’s own late recollection, he racked his brains for days seeking the final law of arranging the elements without finding it, until, dozing off after work at his desk, he clearly saw in a dream a completely arranged table of elements, and on waking immediately recorded what he had seen, thereafter making only minor adjustments to a few positions — this anecdote of "insight in a dream" was widely spread through his own repeated telling, though modern historians of science broadly hold it more the fruit of long systematic thought than pure chance inspiration.
The Blank Spaces That Predicted Unknown Elements
In compiling the periodic table Mendeleev boldly left several positions deliberately blank, publicly asserting that these gaps corresponded to as-yet-undiscovered new elements, and predicting from the periodic law the specific atomic weights and chemical properties of these unknown elements; over the mere fifteen years after, gallium, germanium, scandium, and other elements were actually discovered by other scientists, their measured properties astonishingly matching Mendeleev’s earlier predictions, and this series of successful verifications greatly raised the scientific authority and predictive power of the periodic table as a chemical theoretical tool.