In GLC, an inert solid support is needed to keep the liquid phase stationary in the column. In a packed column, such as is used in this experiment, the support usually consists of porous material of siliceous origin (diatomaceous earth or clay), fairly finely granulated to reduce the turbulence of a gas flowing through it. It should be capable of absorbing the liquid on its surface in a thin, uniform film with a loading of up to 50% by weight. The column packing is prepared by mixing the liquid and solid support prior to filling the tube which holds the packing. The tube material is usually glass or metal (e.g. copper or stainless steel) tubing. Unless the sample to be analyzed reacts with it, metal tubing is preferred because of its infrangibility, and because it can be readily wound into space-saving coils. The operating principle of GLC involves introducing the sample to be studied (the solute) into a heated injector tube where it is vaporized and mixed with an inert gas, usually helium, called the carrier gas. The vapor is then swept onto the column where it distributes itself between the gaseous and the liquid phases. The partial pressure of the solute vapor in the gaseous phase depends upon how soluble it is in the stationary liquid phase (the solvent or substrate). The factors influencing the solubility of the solute in the solvent are discussed later in the section on theory. It should be obvious that the solvent must be a liquid with a very low vapor pressure if it is to remain stationary on the column.