Place of publication: London
Collation: 24 p. ; 19 cm.
Notes: First letter dated September 18th, 1886. Second letter dated September 27th, 1886. Publisher's advertisements on p. 21, 22 and 24.
Place of publication: [New York]
Collation: [45]-47 p. ; 26 cm.
Notes: From the library of Dr Moncure Conway. The article was first published in Galaxy, November, 1869 and later published in Modern Thinker, no. 1, 1870.