Heart of oak.

This extension to a small Cambridge college presents an appropriately demure aspect to the street, yet opens cheerfully to the light and activity of the river.

Darwin is, by Cambridge standards, a new and small institution. Founded as a postgraduate college in the 1960s, it saunters along the bank of the river Cam, incorporating three nineteenth-century houses (including that of the family of the great Victorian sage from whom it takes its name). Connecting tissue by David Roberts and (notably) William Howell of HKPA joins the old buildings along a spinal nerve of internal circulation to form a rambling collection of cells which culminate in a hall that acts as a sort of head for the organism and overlooks the main college garden. This, though it has a soothing lawn and some fine trees, is not large -- for the whole site is severely compressed between the river and Silver Street to the north.

The compression is at its most severe at the tail end of the organism, beyond a croquet lawn and a delightfully picturesque mill house by J.J.Stevenson. The site curves and tapers between the tranquil punt pool of the Cam and the noisiest point in the street, where the tourist buses take up and set down their loads of chattering visitors. Here, the college wanted a study centre -- a combination of library and a place in which students could work in an atmosphere rather more informal than that of a rigorously controlled academic book store.

Dixon and Jones won the limited competition for the project by creating a building that is more or less imperforate to the road while opening south to the river and pool. The upper level on this side consists of four open bays of study area over closed computer rooms on the ground level. The open volume is full of air and sunshine. Deep into the space, dappled light is reflected off the water from the white ceiling to blend with cooler luminance from a sealed clerestory over the tall curved bookstack that lines the wall against the street.

When you enter the study centre from an intricate passageway that leads from the college's croquet lawn through the old mill, the immediate impression is of space, light -- and the strong but delicate yellows and browns of new oak. The architects won the competition with a proposal for a softwood structure, but the slump in the UK economy made oak cheaper than before, and it became possible to use oak structurally and as veneers for furniture.

The basic structure is massive, and made of members that have been only partly dried. There is a row of paired columns which support a spine beam from which rafters fan out gently to bear on the clerestory over the street wall, creating a subtly sensuous curved ceiling admirably adapted for reflecting the dappled river light. The structural members were cut and dried specially for the building, but they retain high and variable moisture contents, so that a good deal of movement is to be expected. Roger Hyde of Ove Arup & Partners evolved calculations suitable for dealing with this ancient material in a modern structure, and with the architects evolved a system of stainless steel fixings which will be tightened as the members dry out over the years. To complement the movement of the oak, the brickwork, on to which the building bears at its edges, is in lime mortar, to allow shifting of the internal structure (It also obviates vertical movement joints).

The big oak columns are riven with shakes and cracks. Against this almost medieval carpentry, the paler veneered oak of the built-in furniture makes a joiner-like counterpoint. The distinction is very clear. Where you sit down, everything is gentle, blonde and smooth, but where the structure emerges, it is tough and ruddy. The play between the sensations gives the place much of its character.

Just as the structure is a sophisticated reinterpretation of ancient technique, the ventilation system ingeniously draws on age-old devices with insights from contemporary technology. All the desk level windows on the south side can be opened by the person sitting nearest to them. If, even so, the space becomes too hot, a layer of clerestory windowsis automatically opened on the south side (the north clerestories are permanently sealed against noise). At the same time, the oak Iouvres in the little tower at the east end of the building are raised, allowing a draft of air to be carried up through the volume to the tower; a combination of electronic controls prevents the Iouvres and windows being opened automatically when the weather is too wet or windy.

On the street side, the new work is reticent and understated: the gentle curve of immaculate buff Cambridge brick is topped by the oak and glass clerestory that supports a roof covered in real slates. It is almost like a garden wall which modestly sweeps the eye towards the main body of the college and the old buildings. Only the little ventilation tower, almost horticultural with its funny, Heath Robinsonish oak flaps opening and shutting apparently at random, signals the importance of the building.

On the river side, there is a cheerful openness. The oak and glass level floor is propped and cantilevered from the brick base, which is pierced only by the regular rhythm of the windows of the computer rooms. The effect is rather sportif with the elegant woodwork of the facade echoed in the immaculate joinery of the punts in the pool below. P.D.