I presume that this is the text you've got this sentence from: http://site.douban.com/110434/widget/articles/289710/article/10074446/ The text refers to the gaps in the periodic table which had been left for elements that had not yet been discovered. Even though Mendeleyev didn't know exactly what these elements were, he was able to work out what some of their properties would be, based on the properties of the known elements in the table surrounding the gaps.