Lethal Witness, By Andrew Rose

The British courtroom in the early decades of the last century was a theatre of shabby stories. They were set in a dingy world of cheap lodgings and mean spirits; their plots were hackneyed and rarely plausible. Their props were household ironmongery snatched up in rage and thrown away in panic, dishevelled undergarments, heavy trunks; the business often concluded at another venue, by rope and trapdoor. In this dismal procession the rouge and "scented booklet of papier poudré", discovered upon the person of a young man posing as a cadet and involved with an indiscreet brigadier, seem dizzily glamorous.

Such flashes of style were usually the preserve of the grand advocates, operatic in bombast and sometimes in bulk, who would loom over their lowly prey like a Zeppelin descending upon a tram depot. Their theatre depended upon more than oratory and flourishes, though. At key moments they called upon a player of a very different style. The expert witness was the voice of authority: concise, level and certain. When the pathologist Bernard Spilsbury played the part, lives depended on it.

The dismaying consequence, as Andrew Rose demonstrates in this accomplished biography, was that people swung because Spilsbury subordinated his science to his art. At a time when the police had only the haziest notion of a crime scene, the pathologist was the dominant source of forensic evidence. Spilsbury made the most of his monopoly. He raised the status as well as the profile of forensic pathology, paving the way for proof based on science.

But what mattered to him most, and to those hearing his expert opinion, was his personal authority. Judges and juries believed him because he never let the mask of certainty slip. On occasions, it would appear, defendants innocent of murder took the truth to the gallows.

Spilsbury's style, dry and terse, projected an air of clarity even when he was obscure or evasive. It pronounced that he was a scientist, and disguised his craft as a storyteller. He would take a corpse and work it up into a narrative of assault and death, revise details, and maintain that his must be the one true story the evidence told. In one case, Rose suggests that he reached the point of perjury.

Rose's assessments are themselves expert opinions, for the author was formerly a barrister and is now an immigration judge. His long experience of defendants and witnesses has not jaded his human sympathies, but he struggles to find something to like in Spilsbury's personality. It is tempting to conclude that Spilsbury worked with corpses because he was indifferent to whether people were alive or dead. Lethal Witness will be a guilty pleasure for aficionados of the era in which body parts were wrapped up in paper and string, of Dr Crippen and the Brighton Trunk Murders. It is more than period drama, though. It is an impressive exploration of a chapter in a longer story, of courtroom science as a drama of authority, that is far from over.

Marek Kohn's 'A Reason for Everything' is published by Faber & Faber

Sutton £20 (288pp) £18 (free p&p) from 0870 079 8897