HF The History of Photography 600pág

Photochemical investigations
3 l
The first person clearly to distinguish between the action of light and heat on
silver salts was JOHANN HEINRICH SCHULZE, professor of anatomy at the University Pl l 5
of Altdorf near Nuremberg.fin 1725, while trying to make phosphorus, Schulze
saturated chalk with nitric acid which happened to contain a little silver. He performed
the experiment at an open window into which the sun was shining brightly, and
to his astonishment noticed that the part of the precipitate facing the window turned
purple, whilst the portion turned away from the light remained white. This phenomenon
struck Schulze so forcibly that he put aside his original experiment in order
to investigate its cause. Tests by the fire proved that it was not due to heat. Schulze
then divided the mixture into two lots, one of which he kept in the dark, exposing
the other to sunlight, with a thin cord tied round the bottle. When, after some hours'
exposure, the cord was removed, he was delighted to find that the liquid under it
had remained the same colour as that in the back part of the bottle, which no ray
of the sun had reached. Schulze then refilled the bottle with the liquid kept in the
dark, and covered it with paper, from which he had cut out words and entire
sentences. 'Before long I found that the sun's rays on the side on which they had
touched the glass through the apertures in the paper, wrote the words or sentences
so accurately and distinctly on the chalk sediment, that many people curious in such
matters but ignorant of the nature of the experiment, were led to attribute the result
to all kinds of artifices.'
Believing that the darkening was attributable to the mixture of chalk and nitric
acid-essential ingredients in making phosphorous by Balduin's process-Schulze
made up a fresh mixture, but failed to repeat the effect. It was some time before he
remembered that the first lot of nitric acid had contained a little silver. He then
prepared a stronger silver 'mixture, adding chalk as before, and the discoloration
took place much more rapidly-and was darker the larger the quantity of silver used.
He also found that chalk could be replaced by other white powdery substances, the
function of which was simply to make the dark words stand out by contrast, and that
the effect was caused not only by direct sunshine but also by light reflected from a
mirror or white wall.
Schulze communicated the details of his experiments to the Imperial Academy at
Nuremberg6 under the title 'Scotophorus pro Phosphoro lnventus' -a little joke
turning on the paradox that the sun should produce darkness : Schulze had been
looking for phosphorus, 'bringer of light', and discovered instead 'Scotophorus',
'bringer of darkness'.
It is evident from Schulze's posthumously published Chemische Versuche7 that he
attached considerable importance to his discovery of the light-sensitivity of carbonate
of silver. There he mentions that if a silver solution is spread on the skin, on wood,
or on bone, and exposed to the sun, it will turn black ; and he stresses once more the
importance of his discovery that the sun's light has chemical effects independent of
heat. Nevertheless, Schulze's observation being incidental to other experiments, he
did not seek any practical application of it. It did not occur to him to spread nitrate
of silver on paper, for instance, and try to produce images by laying on stencils, or
perhaps leaves, though even that would not have entitled him to be called 'the first
photographer'. Professor Eder, who first drew attention in modern times (1881) to
these experiments, called Schulze 'the inventor of photography', and in spite of
numerous protests persisted in his claim in the various editions of his History of
Photography. Of course, if we translate 'photography' literally as 'light-writing', Eder
was right, for Schulze wrote words and sentences in the solution in his bottle, but the
term 'photography' was introduced in 1839 specifically to denote the making of