HF The History of Photography 600pág

2 Photochemical investigations
It is a common fallacy that the blackening of silver salts by light was known to the
alchemists of the Middle Ages and Renaissance. Those who cite Albertus Magnus
(1193-1280), Georgius Agricola (1490-1555), Georgius Fabricius (1516-71), and
Johann Rudolf Glauber (1604--68) as proof of their contention, can have little or no
acquaintance with their works.
ALBERTUS MAGNUS mentions that nitrate of silver 'colours the human skin with
a black colour very difficult to remove', but without attributing the effect to the
action of light. AGRICOLA makes no reference to any of the silver salts at all.
FABRICIUS1 describes for the first time natural silver chloride-argentum cornei or horn
silver, so called because it had the transparency of horn-and remarks that it is the
colour of liver, soft like lead, and melts over a candle flame; but he says not a word
of its tendency to change colour in light. The erroneous idea that Fabricius observed
this is due to Frarn;:ois Arago, who in his report on the daguerreotype process on
3 July 1839 added, 'This substance, exposed to light, passes from yellowish-gray to
violet, and by prolonged action, almost to black.' The statement, coming from the
mouth of such an eminent authority, has been generally accepted without question
in spite of the fact that several historians have since pointed out Arago's error.
There are several passages in GLAUBER ' s Opera chymica (1658) relating to the use of
nitrate of silver solution for staining hard-woods to look like ebony, and for dyeing
leather and feathers black, but he also was apparently unaware of the cause of the
change of colour.
In fact, the only two experimenters before Schulze who attributed the darkening
of silver salts to the sun were Angelo Sala and Wilhelm Hornberg. Sala wrote, in
a pamphlet published in 1614,2 'When you expose powdered silver nitrate [lapis
lunearis] to the sun, it turns black as ink.'
Wilhelm Hornberg on 4 September 1694 exhibited to the Academie Royale des
Sciences in Paris3 among other things a small marbled box made of beef-bone.
Having dipped the bone in a solution of nitrate of silver and blackened it by exposure
to the sun, he gave it a marbled pattern by laying bare parts of the whitish bone
beneath the blackened surface.
It remains a speculation whether either of these two scientists realized that the
blackening of silver was due solely to the sun's light, and not to its heat. Homberg's
contemporary, Robert Boyle, quite clearly attributed the effect to the air,4 and even
a century later Count Rumford argued that all changes produced in bodies exposed
to sunlight were due to heat and not to light5 -although before this ample proof had
been forthcoming to the contrary.