How was neutral hydrogen in our Galaxy discovered?

The discovery of neutral atomic hydrogen (HI) in the interstellar medium was based on a prediction by H. C. van de Hulst. The story of how Jan Oort suggested that the student Van de Hulst might investigate the question, who then reported his results at a wartime colloquium in Leiden, is well known. Archival research in the past few years has turned up letters and other documents which, while not changing the basic storyline, do throw new light on certain details. From letters to Oort, it is now possible to determine when he first heard about Reber’s observations of radio emission from the Milky Way, and what his immediate reaction was. After the 1944 colloquium, Oort began his quest to detect and study the HI line, despite technical and financial limitations, and the lack of radar know-how in the Netherlands. The Dutch effort achieved success in 1951, but not before the Harvard team of Ewen and Purcell had detected the line some six weeks before. The entire story is a fine example of how research in science is often done, with changes in strategy, unexpected obstacles, new input from others and significant social elements.