THE PRACTICE of adding small quantities of non- petroleum bodies to petroleum products—principally lubricating oils and greases—for the purpose of modifying or improving their natural properties has now beon established for at least eighty years. This technique may be legitimately compared with that of alloying iron, although the resulting modification in the properties of the basic raw material are on the whole less spectacular in the case of petroleum hydrocarbons than with the metal. The benefits conferred are also on a rather different level, with one possible exception : Midgley and Boyd's contribution of tetra-ethyl lead. This additive has been of unquestionable value in the design of more efficient internal combustion engines and at the same time assisted the refiner in the production of high quality motor spirit. There must be very few motorists today who have not heard of TEL, who do not understand its function, and many have a rough idea of its composition.