THE BEGINNINGS OF PHOTOGRAPHY. 353 



BLACK. 



In Dr. Joseph Black's Lectures on the Elements of Chemistry 

 (2, 600), he hiis fully treated of the salts and best known ores of silver 

 and discussed the action of lig-ht in chano-ing- the color of the chloride 

 and permanently staining organic and mineral substances moistened 

 with the nitrate. His explanation is similar to Doctor Lewis's, and he 

 saj'S those bodies to which the solution is a])plied attract the acid 

 from the calcined silver, while at the same time this metal is restored 

 to its metallic state, or made to approach that state, by the action of 

 the light, A\ hich expels from the calx a ([uantity of vital air. Tliis 

 effect of light in this and other similar examples is well known ])y 

 experience, but it is not clearly understood how it is produced. Of 

 the chloride he says that if perfectl}' dr}' and white it will not change 

 its color in air that is also perfectly dry, although accessible to light, 

 and then he discusses Scheele's and Berthollet's experiments upon it. 

 These lectures were written before 1796. 



WILLIAM HEHSCHEL AND KITTP^R. 



In 1800 William Herschel, following somewhat in the footsteps of 

 Senebier, discovered the heat rays beyond the visible red rays, by 

 means of thermometrical observations, and this discovery was followed 

 in 1801 by the almost more important one, so far as photography is 

 concerned, made by J. W. Ritter of the invisible ultra-violet rays and 

 of their strong chemical action upon salts of silver. The first account 

 of these results appears in an extract from a letter from Herren Rit- 

 ter and Bockmann, in Gilbert's Annalen, Volume VII, 1801, page 527, 

 discussing HerschePs results. He (Ritter) says: ""On February 22, 

 I also came upon solar rays on the violet side of the color spectrum 

 and beyond it, and indeed proved it b}^ means of horn silver. They 

 reduce even more strongly than the violet light itself, and the extent 

 of these rays is very great." A further conmmnication was made in 

 the Erlangen Literatur Zeitung, 1801, No. 16, page 121, and a complete 

 account of the investigations was given in a paper read before the 

 Jena Society for the Investigation of Nature, in the spring of 1801 

 (reprinted in the collected works of Ritter, Physisch-Chemisch 

 Abhandlungen, II, 81). It is entitled "Remarks on HerscheFs recent 

 researches on light," and is a most interesting paper, more so as 

 regards the chemical action of the red and violet ends of the spec- 

 trum than for any photographic application. 



As the thermometrical method used b}^ Herschel for showing the 

 extension of the spectrum at the red end would have been useless in 

 investigating an extension of the violet end, where changes of tem- 

 perature are not indicated, Ritter, noting Schecle's observation that 

 horn silver, or nuiriate of silver, darkened much moi-c rapidly in the 



