Diagram of cat in glass bell jar with tubes connected. via Wellcome Images. “Experiment to ascertain the quantity of carbonic acid gas excreted by animals whilst under the influence of chloroform and ether. Taken from the article ‘Narcotism by the inhalation of vapours’ by John Snow. Engraving. 19th Century London Medical Gazette, Friday April 4th 1851″ Scan of 2 d image in the public domain believed to be free to use without restriction in the US.