One of Salas primary areas of study concerned chemical identity and change. In 1610 Sala accompanied Count Johann von Nassau as a field doctor and between 1612 and 1617 he worked in The Hague. His experiments at this time with silver nitrate and silver salts were an important step towards the invention of the photographic process; He introduced the alchemical name "magisterium argenti," or "crystalli Dianae," for silver nitrate, which he also called "lapis lunearis" and described in his Opera medica chimicae' the smelting of silver nitrate. 
In Septem planetarum terrestrium spagirica recensio (1614) he reported that "Si lapidem lunearem pulveratum ad solem exponas instar atramenti niggerimus" (When you expose powdered silver nitrate to sunlight, it turns black as ink), and also its effect on paper; silver nitrate wrapped in paper for a year turned black This discovery of the sun’s effect on powdered silver nitrate was not replicated by then "respected" scientists and was subsequently disregarded as having “no practical application," despite the use of silver nitrate in the practice of alchemy.
Robert Boyle made a similar observation later, but mistakenly believed that the darkening resulted from exposure to air, rather than light.