Copy letter, from Sir John Herschel to William Henry Fox Talbot, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Doubts that isomerism of all elements can be verified experimentally, but notes a number of groups with similar atomic weights that might well fit. Further on vegetable colors in photography. Refers to enclosure of a further dialogue on infinity. 6 April 1841