19 



111 the collodion process the character of the organic matter is yet 

 more obviously changed. Collodion is a solution of gun cotton in 

 alcoholised ether; and the gun cotton, so dissolved, maybe prepared from 

 several substances besides cotton, as flax, paper, sawdust, &c., all 

 having the same chemical composition, and coming under the generic 

 name of lignine. These substances, when acted upon by nitric acid, 

 form a series of substitution compounds, which contain relatively two, 

 three, or five equivalents of nitrous acid in the place of as many 

 equivalents of hydrogen removed. 



The decomposition may be thus represented : 

 From Starch, by strong nitric acid, 



(^ ij O TT -1- 2NO — *-"2*' (■'^18. '-^NO^), O20 



'-^24) "2o> '-'20 — ^2 T ^ixyJi ~ -.^. 



Xyioidine. 



From Lignine, i.e. cotton, flax, paper, etc., by mixed acids, 



O24, ±±20, U20 — Ms + 'JINU^— ; ^^ , . , . — 



Oollodion Ayloidme. 



From Lignine, by strongest nitric acid, 



C,„ (H,„ 5 NO,) O20 



'-^•jd, -tliiii, '-'* 



-H. 



+ 5N0,= 

 Or by the following diagram :— 



Pyroxyline. 



-.H„,_. 



^ 004,11,,, 3NO4, O2 

 (cotton xyioidine.^ 



Lignine = 



Nitric Acid=- 



3H0 



(water.) 



We thus see that gun paper or gun cotton contains nitrous acid, one of 

 the most readily decomposed substances we' know, equally ready to 

 throw off 2 cqs. of o.xygen, and be converted into NO.^, or on the other 

 hand to absorb oxygen and form NOj. We may judge then, a priori, 

 that it is a substance very prone to decomposition. But we need not 

 depend on supposition : we have the direct experiments of Dr. Glad- 

 stone to prove, that of these compounds the first is the most ready to 

 decompose, which it does spontaneously, leaving a gummy mass, and 

 yielding N0< as gas. Still further, he finds that the second compound, 

 the one we employ for collodion, also suffers spontaneous decomposition, 



