It is now certain that no portrait exists of the eminent Smithson Tennant, who occupied the Chair of Chemistry at Cambridge from 1813-1815. Even amongst the collection of miniatures of his great friend, Sir Busick Harwood*, where many of the Emmanuel men of his period are portrayed, Tennant is absent. Given the strong friendship that existed between the two men, it seems clear that Tennant must have been invited to sit for his portrait, but declined.

However, a number of friends wrote about different aspects of his life so that from their words we can call up our own picture: he was tall and slender, with a thin face and light complexion, of striking and agreeable appearance with expressive features and strong marks of intelligence. Several people noted his resemblance to a famous portrait of the philosopher John Locke who had lived a century before him.

Six of the best For our new 'Desert Island Books' column we've decided for this issue to ask two of the departmental secretaries to name their favourites: Heather Thomson, who works for Alan Fersht, has chosen: The Small Woman - Alan Burgess The Moon's a Balloon - David Niven What Plant Where - Roy Lancaster Modern Verse 1900-1940 - Chosen by Phyllis M. Jones The Wild Swans - Jung Chang My Left Foot - Christy Brown

Hidden in the grid below are names of 9 chemical elements, which can be read vertically, horizontally, diagonally, up or down, to left or right. Their initial letters spell a word closely associated with Britain's best university . Answers (by mail or e-mail) to Chem@Cam by 1 September. The first correct answer drawn will win £20 and a copy of The Shocking History of Phosphorus by John Emsley, published by Macmillan. Four runners-up will each receive a copy of the book.

The elements hidden in Chemystery square no.7 were chlorine, osmium, lithium, lead, erbium, gallium, europium and sulphur whose initial letters spell COLLEGES. The winner of the £20 prize and a copy of the book The Shocking History of Phosphorus was John O Toole of the Parke-Davis Research Centre, Cambridge. Four runners-up also receive copies of the book and they are Paul White, Ekk Sinn, Jo Woods and Howard Clase. Congratulations to you all!

Elements of Surprise - No. 8, Tellurium Te, atomic number 52, relative atomic mass 127.6, melting point 450oC, p-block semi-metal. A portrait of tellurium by Glasgow artist Murray Robertson, who is currently working on a project to render portraits of all the chemical elements. The name comes from the Latin tellus meaning Earth, which is oddly humble for an element which owes its history and many of its associations with gold. Its discoverer was Müller von Reichenstein in 1782, who was investigating gold ores and it is often found as calaverite, which is gold telluride. Tellurium was first given the name aurum paradoxium because it did not show the properties predicted for the antimony he was expecting. More illustrious are the names of its mineral ores, such as the bright blue, translucent quetzalcoatlite (zinc copper tellurite hydroxide) named after Quetzalcoatl, the Mayan and Aztec feathered-serpent god, and Tlalocite, named after Tlaloc, the rain god. Both of these ores are found in the Moctezuma area of Mexico. Tellurium is a silver-grey, non-metal in group 16. It forms helical chains linked together by metallic interactions. It is a p-type semiconductor whose conductivity increases slightly in light and it is used in photovoltaic cells. Indeed some Cd/Te cells have been claimed to be twice as effective as silicon cells. Tellurium is also used in infrared sensors. When added to steel it improves machinability and resistance to vibration and fatigue. It is also used in rubber as a curing agent, and in glass as a colouring agent.