2022 Reprint of the 1928 Edition. Exact facsimile of the original edition and not reproduced with Optical Recognition Software. Mescaline is the principal active psychedelic agent of the peyote and San Pedro cacti, which have been used in Native American religious ceremonies for thousands of years. A German pharmacologist, Arthur Heffter, isolated the alkaloids in the peyote cactus in 1897. This included mescaline, which he showed through a combination of animal and self-experiments was the compound responsible for the psychoactive properties of the plant. In 1919, Ernst Späth, another German chemist, synthesised the drug. Although personal accounts of taking the cactus had been written by psychologists such as Weir Mitchell in the US and Havelock Ellis in the UK during the 1890s, the German-American Heinrich Kluver was the first to systematically study its psychological effects in this small book called Mescal and published in 1928. The book stated that the drug could be used to research the unconscious mind.