The dream showed him a pattern that he had not noticed before: when the elements were ordered by increasing atomic weight, their properties repeated periodically. He realized that this was a natural law that governed the behavior of the elements. He also left some gaps in his table, predicting that new elements would be discovered to fill them. He even predicted some of their properties based on the patterns he observed.
Handwritten manuscript of the arrangement of elements written by Mendeleev, ca. 1869. Public domain image. Wikimedia Commons.
Mendeleev’s dream was not a random occurrence, but a result of his subconscious mind working on the problem that he had been trying to solve for a long time. His dream was based on his previous knowledge and observations of the elements, but it also revealed something new and unexpected.