195 



at which the process of preparing the article was disclosed 

 in this country. 



Dr. Bacon stated that he had examined, under the micro- 

 scope, specimens of Gun cotton prepared by Dr. C. T. Jack- 

 son, and also some prepared by himself. 



Specimens of the Cotton, before and after preparation, were 

 put up in Canada balsam on slips of glass, and covered by very 

 thin glass. When viewed by transmitted light, with powers from 

 150 to 800, many of the fibres of the Gun cotton appear thick- 

 ened, but no other change can be perceived on comparison with 

 the unprepared article. There is no appreciable difference in 

 the transparency of the two. 



They were now examined in polarized light by means of the 

 polarizing attachment to the microscope. ^Vhen the polarizing 

 and analyzing prisms are so arranged as to afford a dark field, 

 the riband-like fibres of the cotton before preparation are seen as 

 luminous objects upona'black ground, and are tinged with bright 

 and varied colors. They are thus proved to possess a strong 

 polarizing power. The Gun cotton, under the same circum- 

 stances, presents an entirely different appearance. Its fibres are 

 much less luminous, and have a nearly uniform dull blue color. 

 It is evident that the process of preparation has so altered the 

 structure of the fibres as to lessen very greatly their action on 

 polarized light. 



Gun cotton prepared by Dr. Jackson by immersion for twelve 

 and for eighteen hours in the strongest acids, has not lost its po- 

 larizing power in any appreciably greater degree than after an 

 immersion of three minutes only. This agrees with the results 

 of other modes of trial in indicating that the latter period is suffi- 

 cient for the complete preparation of the Cotton, when the acids 

 are of full strength. In all the specimens there are some fila- 

 ments so nearly destitute of polarizing power as to be scarcely 

 visible on the black ground, but none have been found entirely 

 without action. When the polarizing and analyzing prisms are 

 in such a position as to give a bright field, a portion of the fibres 

 becomes tinged with a color approaching to orange, while the re- 

 mainder appear colorless as in ordinary light. 



