Photo graphical Section Pr o ceebings. 1031 



'■'■Uranium salt for positive pHnting and measuring the actinic force 

 of direct s^mlight. — The positive print was obtained by sensitizing 

 the paper with the ox^-fluorides of uranium and potassium, and 

 formic acid. The print was made by Dr. H. C. Botten, who was 

 the inventor of the process. The specimen was the first and only 

 one taken by this process, and was very indistinct. It was brought 

 forward at this early stage in order to fix the date of the inveutioiL 



"The picture is composed of the green fluorides of uranium and 

 potassium, and is permanent. Formic acid produces no precipi- 

 tate in a solution of oxyfluorides of uranium and potassium; but 

 if the acidified solution be placed in the direct rays of the sun, 

 decomposition begins immediately, and a green precipitate of the 

 fluorides of uranium and potassium falls. The precipitate is quite 

 insoluble in water and dilute acids, and could be employed to 

 measure the actinic force of the direct sunlight. 



" The sensitizing bath is prepared by adding a few drops of formic 

 acid to a tolerably concentrated solution of oxyfluorides of uranium 

 and potassium. The paper, while still wet, is placed upon the 

 negative and exposed to the rays of the sun fifteen minutes." 



Prof. Tillman called attention to recent discoveries by M. Pratt, 

 of Paris, who claims to have isolated fluorine, the only chemical 

 element before unknown in a separate state. He has made, by the 

 direct combination of fluorine gas with silver, a fluoride of silver, 

 which is more sensitive to light than chloride of silver. The ordi- 

 nary fluorides are regarded by him as oxyfluorides. Upon this 

 assumption, the combining proportion of fluorine is 29.5, instead 

 of 19. 



Prof. Tillman stated that the theoretical deductions which con- 

 firmed M. Pratt's statement that fluorine is a colorless gas, would 

 lead us to place combinations containing fluorine lowest in the list 

 of halogen compounds which are influenced by the actinic rays. 

 Prof. Tillman had, at a previous meeting, stated some facts in rela- 

 tion to fluorides, and could not at present coincide with M. Pratt, 

 but would at some future time give more information on the subject. 



Prof, Tillman alluded to a statement made at a meeting of the 

 Astronomical Society of London, that in the photographs of the lunar 

 eclipse which occurred on the 13th of September last, a portion of 

 the imobscured part of the moon Avas absent. M, De La Euo 

 stated at that meeting, in relation to the nature of this phenomenon, 

 that more of the moon was eclipsed chemically than optically, 



Dr. Boynton said the interesting peculiarities noticed by British 



