The elements which fall into group IVA of Mendelejeff’s classification are titanium, zirconium, cerium, and thorium; the elements of lower atomic weight, carbon and silicon, are placed by some authors in Group IVB, by others in Group IVA. It is a feature of the periodic system that the members of the A and B families show great differences in the end groups, I and VII, II and VI, which disappear as the middle groups are approached; in group IV the families A and B show only slight differences in properties, corresponding to their amphoteric character and electrochemical indifference, so that the elements carbon and silicon may be placed as well in the one as in the other. Generally they are placed in family B. In its tetravalent condition, titanium shows a close relationship to silicon; the similarity is manifested by the ease with which the dioxide replaces silica in many minerals, and the isomorphism of many titanates with corresponding silicates. Yet the strengthening of electropositive character, which always accompanies the change in atomic weight in descending a vertical column of the table, is very apparent in the case of titanium, and its ability to form salts in the tetravalent state is a very important property. This strengthening of the electropositive character is still more marked in the case of the succeeding elements. The salts of zirconium are highly hydrolysed in solution, but they are considerably more stable than those of tetravalent titanium; the ceric salts show the same change, whilst thorium salts are comparatively stable in solution, and can be recrystallised from water without change. Zirconium hydroxide will not dissolve in alkalies, though zirconates may be obtained in the dry way; thorium hydroxide shows no acidic properties whatever. The change in electrochemical character is accompanied by corresponding changes in physical properties of the elements and their compounds. With the exception of cerium, which has a very low melting-point (623°), the elements fuse only at high temperatures; titanium is the most refractory, zirconium melts at over 1500°, and thorium at about 1450°. The boiling-points of the chlorides rise as the series is descended; titanium tetrachloride boils at 136°, zirconium and thorium chlorides at 400°-450° and 950° respectively; zirconium chloride partly sublimes, whilst ceric chloride decomposes when heated.