How did Mendeleev compare the halogens with the alkali metals?

When Dmitri Mendeleev was composing the chapter on halogen elements for his textbook, The Principles of Chemistry, he compared the properties of the halogens to those of the alkali metals, such as sodium.1

Despite these two groups consisting of dissimilar elements, Mendeleev discovered similarities in the progression of their atomic weights. This observation led him to wonder if other groups of elements exhibited similar properties, prompting him to study the alkaline earths as well. Ultimately, Mendeleev established that the order of atomic weights could be used to arrange both the elements within each group and the groups themselves, a discovery that served as the foundation for his formulation of the periodic law.1