48 PROPERTIES OF OXALIC ACID. 



many lichens, especially of lheparmeli(S and variolariee,* some of which 

 contain as much oxalate of lime as is equivalent to 15 or 20 parts of pure 

 acid in 100 of the dried plant. 



The crystallized oxalic acid of the shops forms transparent colourless 

 crystals, of an intensely sour taste. These crystals dissolve readily in 

 twice their weight of cold water, and the solution, when sufficiently di- 

 lute, is agreeably acid to the taste. This acid is exceedingly poisonous. 

 Half an ounce of the crystals is sufficient to destroy life in a very short 

 time, and a quarter of an ounce after the lapse of a few days. It con- 

 sists solely of carbon and oxygen in the proportion of two equivalents 

 of the former to three of the latter. Its symbol is C2O3. It combines 

 with bases, and forms salts which are known by the name of oxalates, 

 and it is characterised by the readiness with which it combines with lime 

 to form oxalate of lime. If a solution of the acid be poured into lime wa- 

 ter, the mixture immediately becomes milky from the formation of this 

 compound, which is insoluble in water.f It is this oxalate of lime which. 

 exists in the lichens, while oxalate of potash exists in the sorrels. 



Oxalic acid is one of those compounds of organic origin which we can- 

 not form, as we can form carbonic acid by the direct union of its elements. 

 In all our processes for preparing it artificially, we arc obliged to have re- 

 course to a substance previously organized in the living plant. It may 

 be prepared from sugar, starch, or even from wood, by various chemical 

 processes. The usual method is to digest potato starch with five times its 

 weight of strong nitric acid (aquafortis), diluted with ten of water, till red 

 fumes cease to be given oflti and then to evaporate the solution. The ox- 

 alic acid separates in crystals, or, as it is usually expressed, crystallizes in 

 the solution thus concentrated by evaporation. 



It is not known to exist in the soil or in the waters which reach the 

 roots of plants. Where it is found in living vegetables, therefore, it must, 

 like the other substances they contain, have been formed or elaborated 

 in the interior of the plant itself. By what very simple changes the 

 production of this acid is or may be effected, we shall see in a subse- 

 quent lecture. 



§ 3. Carbonic oxide, its constitution and properties. 



When carbonic acid (CO2) is made to pass through a tube containing 



red-hot charcoal, it undergoes a remarkable change. Its gaseous form 



remains unaltered, but it combines with a second equivalent of carbon 



(becoming C2O2), which it carries off'in the aeriform state. The new 



' The parmelia cruciataa.nd variolaria communis are mentioned as peculiarly rich in this 

 acid, which used to be extracted from them for sale. A species of parmelia, collected after 

 the droughts on the sands of Persia and Georgia, contains 66 per cent, of oxalate of lime, 

 with about 23 per cent, of a gelatinous substance similar to that obtained from Iceland moss. 

 This lichen is used for food by the Kirghuis. A similar lichen is collected about Bagdad for 

 a similar purpose. 



t Substances that are insoluble are generally without action on the animal economy, and 

 may be introduced into the stomach without producing any injurious effect. Hence tliis ox- 

 alate of Ume, though it contains oxalic acid, is not poisonous. Hence also, if oxalic acid be 

 present in the stomach, its poisonous action may be taken away by causing lime water or 

 milk of lime to be swallowed in sufficient quantity. The acid combines with the lime, as in 

 the experiment described in the text, and forms insoluble oxalate of lime. The common 

 magnesia of the shops will serve the sarae purpose, forming an insoluble oxalate of magnesia. 

 It is by performing experiments under circumstances where the results are visible — as in 

 glass vessels — that we are enabled to predict the results in circumstances where (he phe- 

 nomena are not visible, and to act wif;h as much confidence as if we could really see them. 



