Gmelin is divided into 71 sets of volumes, each covering an element or, for minor elements, a group of elements, e.g. rare earths. Each set has one or more main volumes and usually a number of supplemental volumes covering new information. Each element (or group of elements) is assigned a unique System Number which has no relationship to its atomic number. The numbering sequence starts with rare gases and progresses to the heavier metals. The system numbers are assigned so that the elements which commonly form cations have a higher system number than those which commonly form anions. Thus, the system arranges compounds under the most electropositive element. To find the system number for an element, consult the periodic table on the inside cover of any of the Formula Indexes. (Lockwood Science & Engineering Reference QD 151 G632).
Obviously, all compounds contain 2 or more elements. Each compound is published in the set for the element with the highest system number. For example, UOCl2 is published in the Uranium volume, since the system number for U is 55, for O is 3, and for Cl is 6.
The Gmelin volumes are organized on the Lockwood Science & Engineering Reference shelves in alphabetical order according to element symbol even though each volume is labeled on the spine by the system number. Thus, system numbers on the shelf are not in order. It is important to first find out from the Formula Index what the element symbol and system number is before approaching the volumes. To find the system number for an element, consult the periodic table on the inside front cover of any formula index. The Gmelin system number for each element is shown in green. Note: the yellow inserts on the shelf tell you where the element group begins. The supplemental works have the abbreviation of the element on the spine of each volume.
