SPECIAL METHODS. 155 



like gum arabic. Part of these substances are formed from 

 cellulose, but most of them are formed by the plant directly 

 as mucilages. It should be observed that the occurrence of 

 vegetable mucilages and gums within the plant is not at all 

 restricted to the cell-wall, but they may also be formed 

 within the protoplasm. But it has seemed to me best to 

 discuss all these bodies together here, in view of their un- 

 doubted relationships, which may lead one, with Beilstein 

 (I, 877), to group them under the designation gums. 



274. So far as the chemical relations of the gums are con- 

 cerned, it should first be observed that most of them, so far 

 as they have been analyzed, agree in their percentage com- 

 position with cellulose and thus correspond to the formula 

 C 6 H 10 O & . But, on the other hand, they differ considerably 

 from cellulose in their chemical relations, and also show 

 great differences among themselves. 



Thus some of them are colored blue by iodine alone,, 

 others only by iodine and sulphuric acid or chloro'iodide of 

 zinc, and still others are colored only yellow or not at all by 

 iodine preparations. 



A part of the gums are soluble, a part quite insoluble, in 

 cnpr ammonia. 



On oxidation with nitric acid, a part of them give oxalic 

 acid, (COOH) 2 , a part, mucic acid, (CHOH) 4 .(COOH) 2 , a 

 part, both acids. 



Unfortunately the chemical characters of the various, 

 gums are not determined with sufficient exactness to make: 

 possible a strictly scientific grouping of them. But in the 

 following account some remarks on the general methods of 

 recognizing the gums may be in place, and then the chief 

 chemical characters, and especially the microchemically 

 applicable reactions, of the gums which have been studied 

 in detail may be brought together. 



275. For the microchemical recognition of the gums their 

 strong power of swelling- in water may first be used. To 

 follow the process of swelling exactly with the microscope, 

 one may first place the objects in absolute alcohol, in which 

 all the gums are insoluble and do not swell, and then 



