281 



cases, fresh rain-water, but then only such as has been caught 

 directly from the sky, and not by means of spouts. 



Water is also employed to withdraw from the ethereous, alco- 

 holic, (fee, extracts, gum, protein substances in their soluble foi'm, 

 acids, most of the alkalies and alkaline earths in combination with 

 organic acids, most of the salts of the alkaloids, some alkaloids in 

 the free state, many bitter substances, some other indifferent 

 matters, pigments ; while chloro]>hyll, wax, fats, resins, certain pig- 

 ments are not dissolved by water; ethereal oils, certain alkaloids, 

 bitter principles, other indifferent substances, some pigments dis- 

 solve in it only sparingly. 



Acids are not used as solvents in the concentrated state, but as 

 addition to water, chiefly in order to dissolve pectin and oxalate 

 of lime. As a rule the diluted acids {hydrochloric, sidphtiric) are 

 only applied after the vegetable substance has been exhausted 

 successively by ether, alcohol and water. 



In cases where the search for alkoloids and their isolatioii with- 

 out loss is to be instituted, water mixed with only 2 to 3 per cent, 

 of the concentrated acids is employed, but even in this diluted 

 state the acid acts not only as a solvent but also altering and 

 decomposing, for instance, on glucosids, and even on some 

 alkaloids. 



Acetic acid of about 20 per cent, is em^iloyed for separating 

 oxalate, phosphate, and sulphate of lead, which are insoluble in 

 the reagent, from the compounds of lead with other organic acids, 

 which are then re-pi"ecipitated from their solutions on addition of 

 a base. 



The fixed caustic alkalies likewise ai-e only used in a very diluted 

 state, and are always preceded by ether, alcohol, water, and acids. 

 Their usefulness is vexy limited, because in most cases and by their 

 agency only those protein substances are dissolved which are in- 

 soluble in water, while again the partial decomposition of these is 

 very difficult to avoid. 



Another inconvenience connected with these alkalies consists 

 in the conversion of so-called extractive substances, and of such as 

 have not been dissolved by the other extracting agents into partly 

 humus-like dark-coloui"ed products, which greatly impede the 

 further process of the analysis. The only important use of diluted 

 alkalies is to soften the vegetable fibre to such an extent as to 

 facilitate its purification. 



Liquor of ammonia as a solvent is only employed for treating 

 complex substances after they have been withdrawn from the 

 plant, in order to remove one or other of the constituents, 

 especially resins, some of which are soluble in liquor of ammonia, 

 others not. 



All the other above-mentioned chemicals — benzol, chloroforin, 

 wood-spirit, sidphide of carbon, petroleum, oil of turpentine, &c. — 



