Like Paul De Kruif and Hans Zinsser, Jurgen Thorwald has the ability to reach and teach science to an audience of laymen through well told, documented, narrative histories. In this book, his technique is essentially the same as it was in his The Century of the Detective, the popular history of criminology in our time. First he gives a careful account of the crime and follows with a clear explanation of the laboratory breakthrough that allowed a blood stain, a hair root, dust or a microscopic liver cell to point to a murderer. The greatest part of the book is devoted to the series of increasingly elaborate tests in the chemistry of blood, which have always had forensic as well as medical significance. Each test accepted as reliable evidence since the late 19th century to the present is introduced through the crime it helped solve. Mr. Thorwald's history has a point. He presses for better police training in the requirements for gathering, preserving and submitting materials for chemical analysis and he records a number of frustrating contemporary cases in this country, where police or coroner ignorance of chemical procedures resulted in miscarried justice. There is an extensive bibliography. Who-dun-it and which-microscopic-scrap-hung-'em--an unbeatable combination for the reader of criminal non-fiction.