Engraving depicting an Louis Pasteur experiment demonstrating that fermentation and putrefaction are caused by air-borne organisms. The contents of the flask are sterilised and the flask is then left with the S neck open; but any organism entering falls into the bend in the S. It is only when the neck of the flask is broken and air admitted that any change takes place. Louis Pasteur (1822-1895) a French biologist; microbiologist and chemist renowned for his discoveries of the principles of vaccination; microbial fermentation and pasteurisation. Dated 19th century.