CONSTITUENTS OF COTTON-FIBRE. 119 



dinary kinds, and the colour is for the most part removed 

 by treatment with caustic alkali. 



Pectic Acid. 



A considerable portion of the organic matter extracted 

 from cotton by caustic alkali consists of a body belonging 

 to the pectine class. Which member of this class it is 

 that exists originally in the fibre is a question on which I 

 express no opinion, though there can be no doubt that the 

 precipitate produced by acid in the alkaline extract of cot- 

 ton contains pectic acid itself. This acid, which is inso- 

 luble in water, is almost entirely converted, during the pro- 

 cess of purification which I adopt, into an acid soluble in 

 water, which seems to be identical with Fremy's parapectic 

 acid. The chief properties of this soluble acid are as 

 follows : — 



On evaporation it is left as a light-yellow, amorphous, 

 translucent substance, resembling gum or gelatine. When 

 heated on platinum it burns with a slight flame, without 

 previously melting, leaving a little ash, which is yellow 

 or brown and only slightly alkaline, and consists chiefly 

 of alumina, together with oxide of iron and lime. The 

 watery solution is clear and colourless and reddens litmus- 

 paper. It gives abundant, white, flocculent precipitates 

 with sulphuric, nitric, hydrochloric, and acetic acids, as 

 well as with baryta- water and acetate of lead. When the 

 watery solution is mixed with several times its volume of 

 alcohol the liquid gelatinizes, the jelly being perfectly clear 

 and transparent. The substance is decomposed with dif- 

 ficulty by boiling nitric acid ; and no oxalic acid can be 

 discovered among the products of decomposition. When 

 treated with strong caustic potash or soda-lye it turns 

 yellow ; and on boiling, the liquid assumes a bright-yellow 

 colour similar to that of chromate-of-potash solution. On 

 continuing the action of the caustic alkali the substance is 



