A Change of Climate by Hilary Mantel Viking $255 HILARY Mantel's latest novel spans two continents and three decades. Its central characters are a Norfolk couple Ralph and Anna Eldred who shortly after their marriage sail to South Africa and a new life as missionaries in the black township of Elim on the outskirts of Pretoria. The year is 1956 and the Nationalist Government is refining its policy of apartheid with increasingly draconian legislation. Ralph and Anna are typically well-meaning English people abroad. A little education, a kind word, a cup of tea, tolerance - these are all you need to make the world a better place. They are incapable of understanding such a complex country with its long history of racial and tribal hatred. Koos, a Boer doctor exiled to Elim in disgrace after his affair with a coloured girl, tells Ralph: ''Do you think that while you've been in this township you have learned anything at all about what goes on here? Because if you do, man, you're more of an imbecile than I took you for.'' After their involvement in a protest against rising bus fares in Elim, the Eldreds are forced out of South Africa and are sent north by the mission society to a remote outpost in Bechuanaland. There they find the environment and the people even more of a mystery than the township, and their inability to comprehend the deeply superstitious and primitive culture which surrounds them leads to a tragedy from which they never really recover. They return to Britain. Ralph continues his mission work, co-ordinating a charitable trust set up by his devoutly religious and overbearing father, while Anna brings up their four children. Although they get by and their daily routine seems that of a normal middle-class family, Africa has changed them irrevocably and as the years pass the changes become more pronounced. Anna sees the alteration in her husband, ''who gave her only glimpses of the gentleness of those early years; she had to look at his sons, as they grew up, to see the kind of man Ralph had once been''. Behind the facade there is a gradual deterioration in their relationship which threatens to result in more tragedy for Ralph and Anna. Hilary Mantel's descriptive prose is powerful. She has a perfect eye for colour and uses the beautiful but bleak landscapes of both the African bush and the Norfolk fens as reflections of the despair inhabiting the Eldreds. She dissects her characters with almost scientific care, peeling away the pretence until reaching the raw nerve of fear. Her portrayal of Anna as a bewildered innocent in a hostile land shows Mantel at her most perceptive. There are times when the dialogue and storyline sag. The conversations between the Eldreds' grown-up children sound stilted and artificial. However, the author's rigorous exploration of Ralph and Anna and the ordeals they endure, give cohesion to this novel which despite its flaws is an impressive and compelling work of fiction.