195. ARABIC ACID. 211 



action of nitric acid may also result in the discovery of distinctive 

 properties. Special attention should be directed to the presence 

 among these oxidation-products of mucic acid, and to the quantity 

 in which it is yielded. 



The action of aqueous solutions of these mucilaginous sub- 

 stances on polarized light also requires further investigation. It is 

 at all events certain that some arabins are powerfully laevo- 

 rotatory (cf. 146), others feebly so, whilst some again are dextro- 

 rotatory. 



By the action of hydrochloric, or tolerably dilute sulphuric 

 acid, or spirit containing 10 per cent, of the latter, arabic acid is 

 converted into metarabic acid ( 226) which is characterized by only 

 swelling in water. This modification can be converted into arabic 

 acid by boiling with a very dilute non-oxidizing acid, a little sugar 

 being simultaneously produced. A similar change to arabic acid 

 takes place when metarabic acid is triturated with sufficient lime 

 or baryta water to dissolve it, a lime or baryta salt of arabic acid 

 being formed. 



Arabic acid agrees in its essential properties with metapectic 

 acid, and approaches pectic acid in such a manner as to allow of 

 the conjecture that the latter, when examined in a state of purity, 

 will prove identical with it ; in fact, I am of the same opinion as 

 Eeichardt 1 and others viz., that the so-called pectin substances 

 are nothing else than the various forms of vegetable mucilage and 

 its nearest allies. 



The insolubility of these mucilaginous and c pectinous ' sub- 

 stances in alcohol and ether, and the property they possess of 

 swelling in contact with water, allow of their detection micro- 

 scopically. They are generally coloured yellow by iodine water 

 (for allied substances coloured blue with iodine, see 244). 

 Aniline violet colours vegetable mucilage red. 



196. Varieties of Gum Arabic. Masing 2 has published com- 

 munications on the behaviour to reagents ( 73) of arabic acid, 

 and different varieties of gum arabic, and its more important 

 surrogates. It appears that 10 per cent, aqueous solutions of 

 these substances are not precipitated by cold saturated solution of 

 acetate of copper, 10 per cent, acetate of lead solution, or by ferric 



1 Archiv d. Pharm. [3], x. 116, 1877 (Journ. Chem. Soc. xxxii. 502). 



2 Archiv d. Pharm. [3], xv. 216, 1879 ; xvii. 34, 1880 (Year-book Pharm. 

 191, 1881 ; Journ. Chem. Soc. xl. 212). 



142 



