280 



Besides these there are many other hygroscopic substances, but 

 none exceeds in activity and cheapness the above three. 



After they have done their service as desiccating agents, the 

 chloride of calcium solution may be evaporated and fused again. 

 The aqueous sulphuric acid may be utilised as such; and the 

 hydrate of lime, as left, would be of no further value. 



B. — Solvents. 



These are principally ethei', alcohol and water; also diluted acids 

 (especially hydro-chloric and sulphuric), diluted alkalies (potash or 

 soda-ley and liquor of ammonia) ; less frequently used are benzol, 

 chloroform, wood-spirit, sulphide of carbon, petroleum, oil of turpen- 

 tine, &c. 



Ether withdraws from the plants, almost without exception, 

 easily and completely chlorophyll, wax, fixed and volatile oils, 

 free acids; also certain alkaloids, pigments, indifferent bitter in- 

 gredients, resins; sparingly or not at all saccharine substances. 

 But even such compounds as are insoluble in pure ether, as humus 

 substances, salts of anorganic acids, proteins, &c., dissolve, though 

 in exceedingly small quantities, in ether containing traces of water 

 or alcohol, and retard thereby the isolation of the first-named sub- 

 stances in their pui'e state. 



This inconvenience might be avoided entirely or for the greatest 

 part by using ether of 0'720 sp. gr. (at 15°), which is entirely free 

 from water and alcohol. 



Of greater consequence still than with ether is the strength of 

 the alcohol (spirit of wine, ethylalcohol) employed as a solving 

 agent. Absolute or anhydrous alcohol of 0-792 sp. gr. (at 15°) 

 dissolves like ether readily chlorophyll, volatile oils, free acids, 

 alkaloids, indifferent bitter substances, resins and pigments; less 

 easily wax, fats and saccharine matters; spai'ingly humus sub- 

 stances, salts of anorganic acids and proteins. 



Yet as it is troublesome to prepare and to keep alcohol in its 

 anhydrous state, it is reserved for special cases, and in its stead an 

 alcohol of 0"815 sp. gr. is generally in use, the solving power of 

 which is nearly the same in regard to wax and fats, and is some- 

 times even greater towards the other substances mentioned. Un- 

 fortunately it also dissolves to some extent gum and similar 

 matters, which have afterwards to be removed by special processes. 



It is not advisable to use weaker alcohol as a solvent, as this 

 would prevent complete isolation of vai'ious constituents. 



Dilute alcohol is employed in some cases for withdrawing resins 

 from fats, or for sepai-ating different resins from each other. 



For extracting purposes, spring or any other ordinary water 

 should not be used but always distilled water, or, in exceptional 



