Laughter is caused by the slackening or relaxation of the nerves. It is an ideal condition and not physical
DAS.Diary notebooks and letters of Mirza Ahmad Sohrab, Dec. 1912 – Jun. 1915. English and Persian diary notebooks and letters held in the U.S. National Bahá’í Archives. Often quoted in other sources such as SW. Catalog entries are indexed by date and include only the English notebooks and letters. Letters from 1 Jul 1913 through 19 Oct. 1914 found here: https://bahai-library.com/sohrab_diary_letters_1913-1914
SW.Star of the West, 25 vols. (1910-1935). Periodical issued each Bahá’í month. Vols. 1-12 reprinted 1978 in 8 volumes by George Ronald. Each issue Includes a section in Persian. https://bahai.works/Star_of_the_West