HF The History of Photography 600pág

32 The prehistory of photography
permanent pictures by the action of light. Yet from Schulze undoubtedly stems the
whole genealogical tree of photochemistry which, combined with the camera
obscura, eventually produced photography.
The first recorded application of nitrate of silver to paper is contained in a communication
in I737 to the Academie Royale des Sciences, Paris, by JEAN HELLOT 8
under the heading 'Sur une nouvelle encre sympathique'. Amongst other methods of
secret writing he mentions the use of a weak silver nitrate solution which is invisible
on white paper so long as it is kept in darkness. On exposure to the sun, the writing
appears in the course of an hour, in a sort of slate colour. Hellot, however, ascribed
this blackening of silver to the presence of sulphur in the nitric acid.
The discovery of the light-sensitivity of silver chloride belongs to GIACOMO
BATTISTA BECCARIA, professor of physics at the University of Turin, who experimented
on this substance in an analogous way to Schulze on carbonate of silver.9
DR WILLIAM LEWIS, F.R.S., of Kingston-on-Thames, repeated and confirmed
Schulze's experiments in his Philosophical Commerce of Arts (I763).10 This would not
in itself be worth recording were it not for the fact that Dr Lewis forms an important
link between Schulze and Wedgwood. The Philosophical Commerce of Arts is not only
the first book published in England in which Schulze's experiment is described, but,
what is far more significant, after Lewis's death in I78I his notebooks containing his
own experiments and compilations from other authors were bought by Josiah
Wedgwood, the famous potter, who also took Dr Lewis's chemical assistant, Alexander
Chisholm, into his service.11
Eliza Meteyard in her biography of Josiah Wedgwood12 supplies the additional
information that Chisholm had been with Dr Lewis for thirty years and entered
Wedgwood's service at Etruria as secretary and chemical assistant in I782. A highly
educated man and a classical scholar, he became Josiah Wedgwood's right-hand man,
and tutor to his youngest son Tom (then eleven years old) in classics and chemistry.
However, in order not to disturb the chronological order of events, we must first
discuss the important photochemical researches of Scheele and Senebier, which
preceded Wedgwood's photographic attempts.
CARL WILHELM SCHEELE, a Swedish chemist renowned for numerous important
discoveries, also carried out experiments on the action of light upon chloride of silver.
These are described in detail in his Chemische Abhandlung13 (I777) , which was translated
into German, English, and French.
Scheele also confirmed that the blackening of silver salts is due to light and not
to heat, and asked himself, 'Could it be that this black pigment is real silver?' To
find out, he spread silver chloride powder on paper and exposed it to the sun for two
weeks, when the surface of the white powder had become black. Knowing from
Glauber and other early chemists of the solubility of chloride of silver in ammonia,
Scheele poured some on the powder and found-as he expected-that it dissolved a
quantity of the chloride of silver, but some black powder remained. This black
precipitate proved to be metallic silver reduced by the action of light (para. 63).
The importance of Scheele's observation is his discovery that the blackened silver
chloride had become insoluble in ammonia, and it is strange that this significant fact
was overlooked by Wedgwood and Davy, whom it would have provided with at
least a partially successful fixing agent. Thus we might have had photography
about I8oo.
Another experiment by Scheele, important for the history of photography, is
described in paragraph 66 of Chemische Abhandlung. Allowing the solar spectrum to
fall on powdered chloride of silver sprinkled on paper, Scheele found that the violet