THE BEGINNINGS OF PHOTOGRAPHY. 339 



system of mineralogy (1T58), of which an Eno-lish translation by von 

 Engestrom and Costa appeared in 1770, and a second edition by 

 Magellan in 1788. He sa3^s that the horn silver ore is the scarcest 

 silver ore; it is of a white or pearl color, changeable on the surface, 

 semitransparent, and somewhat ductile when crude and when melted. 

 It can not be decomposed without some admixture of such substances 

 as attract the combined acid of the sea salt. Although he notes the 

 blackening of the glassy ore or sulphide in the air, he does not clearly 

 mention the blackening of the horn silver. In Magelhin's edition the 

 darkening of this ore to a violaceous brown when exposed to the sun's 

 beams, as happens also to the artihcial horn silver, is mentioned in a 

 note. 



The first definite chemical analysis of crude mineral horn silver 

 compared with the artificially prepared, was made by Peter Woulfe 

 (Phil. Trans., 6(), 1776, 608). He notes the confusion existing between 

 the horny ores and the glassy ores, containing sulphur and arsenic, 

 and says that Cronstedt and Le Sago asserted that the native horn 

 silver was composed of silver and sea salt only. Woulfe also found 

 some sulphate, amounting to about one-third of the chloride, and in 

 some samples admixture of sulphides. He says nothing al)out the 

 darkening in light, but mentions a black horn silver. 



DARKENING ACTION OF SILVER NITRATE. 



On the other hand, the darkening action of silver nitrate was known 

 much earlier. It is generally said thtit Albertus Magiuis was the first 

 to record it in his Compositum de Compositis, but this is not correct. 

 After his description of the preparation of nitric acid, which is very 

 similar to Geber's, he says: "It dissolves silver and separates it from 

 gold; it calcines mercury and crocus martins; it stains the human 

 skin with a black color diflicult to remove." As given in the texts 

 this latter passage certainly refers to the acid and not to the solution 

 of silver. But there may have been some silver in the acid. 



In J. B. Porta's Magia Naturalis (1589) liber 10, caption 20, we find 

 a modification of Ge))er\s old recipe for making aqua fortis, for part- 

 ing silver from gold, by distilling niter and alum, also for making 

 aqua regia by adding sal ammoniac to the other ingredients, and another 

 for sulphuric acid. In liber 16 he gives a number of methods for 

 secret writing, among others writing on the skin with a solution of 

 silver in aqua fortis, and in liber 20 he gives a method of disguising 

 oneself for some time by applying the same solution over the body. 

 It is curious that he should not have thought of applying the solution 

 to paper. 



In the well-known work by Caneparius, De Atramentis (161i»), deal- 

 ing with the preparation of pigments and inks of various kinds, we 

 might have expected to find mention of the use of a solution of nitrate 



