The discovery of the therapeutic effects of lithium is one of the most fabulous examples of serendipity in all of science. In 1949, John Cade was a 37 year old superintendent of a psychiatric hospital in the Australian outback. Cade was intrigued with the theory that mania was caused by the overproduction of some biological “substance.” To investigate this, he injected the urine of manic patients into guinea pigs. All of the animals died, and Cade speculated that crystallized uric acid might be the culprit. So he ordered a substance to help keep the uric acid safely dissolved. This substance was the alkali metal lithium, which had been used for various conditions in medicine (including gout) since the 1850s. Guinea pigs injected with lithium urate survived, and on a whim, Cade injected them with lithium alone (combined with carbonate). The animals became apathetic and sedated. He decided to try lithium on his manic patients, and the response was dramatic.
Source: Edward Shorter, A History of Psychiatry, New York: John Wiley and Sons, Inc., 1997.