122 Trans. Acad. Sci. of St. Louis. 



sodium hyposulphite. The time of exposure when only iodin e 

 was used in the preparation of the film was from three to 

 thirty minutes, but this was very much shortened by the use of 

 bromine in 1844. It was then possible to take so-called in- 

 stantaneous views of well-lighted objects. A little later 

 iTizeau treated the level plate with a solution of gold chlorid e 

 mixed with sodium hyposulphite, which was warmed over a 

 lamp until the plate had received the re-enforcement possible 

 with this process. This produced a picture in slight relief , 

 and most of the plates extant are of this class. 



The wet plate collodion process which followed in 1850 

 possessed the advantage that prints could be made from the 

 original negative, and that these prints show the object cor- 

 rectly as to right and left, which was not the case with the 

 daguerreotype. Dry plates were first shown to be possible in 

 1854 by Gaudin. The dry-plate first became practically an 

 assured success by the introduction of the alkaline developer, 

 with films made sensitive by means of bromide and chloride 

 of silver. This improvement is said to be of American 

 origin, prior to 1862, but neither the date nor the author 

 seems to be known. Up to 1880, pyrogallic acid was the sole 

 reducing agent in the alkaline developer, but in that year 

 Captain Abney discovered that hydrochinone was a most ef- 

 fective agent. From that time many other developers have 

 been used. As an all-around developer pyrocatechin is prob- 

 bably the best yet discovered. There is a possibility that the 

 future may see the elimination of the dark-room and the 

 negative from photography, and the direct printing of positives 

 from positives with short exposures. 



If the history of the last century has taught us anything, 

 it has established the practical or commercial value of research 

 in pure science. It is from such work that all of the great 

 achievements have directly come. And whenever any people 

 forgets the source from which these great things have come, 

 and allows engineering to supplant science, that people is on 

 the way to the civilization of China. There are great prob- 

 lems yet to be solved. The burning of coal is a mere inci- 

 dent in human history. There are men now living who can 

 remember when its use began, and there are boys now living 



