180 PRACTICAL LESSONS IN SCIENCE. 



9 



oxalic acid occurring in sorrel, rhubarb and other plants. It 

 is prepared by mixing sawdust into a paste with a strong solu- 

 tion of the potassium and sodium hydrates, and heating the 

 mixture on iron plates. The wood fiber is converted into oxalic 

 acid, which combines with the potassium and sodium forming 

 oxalates, from which the pure acid is obtained . Oxalic acid is 

 very poisonous ; the best antidote is chalk or magnesia. It is 

 much used in calico printing, in cleansing articles of brass and 

 copper, in removing stains of iron rust or ink and as a solvent 

 of Prussian blue in the manufacture of blue ink. 



Wine during fermentation deposits on the inside of the cask a 

 crust called argol. It is a hydrogen potassium tartrate, and 

 when purified it is called cream of tartar. From argol, or cream 

 of tartar, tartaric acid (C 4 H 6 6 ) is prepared by the aid of lime 

 and sulphuric acid. It occurs in many fruits besides the grape 

 and is extensively used in dyeing and calico printing. It is also 

 used in medicine, as Eochelle salts and tartar emetic. 



The acid of lemons, oranges and some other acidulous fruits, 

 called citric acid, is used in the preparation of acidulated drinks. 

 It is also used in medicine and in dyeing operations. 



In the bark of the oak, pine, hemlock and other trees, in the 

 roots of many plants, in tea, coffee, and many other vegetables 

 is found an astringent substance called tannin or tannic acid. 

 Tannic acid may be extracted from its various sources by water, 

 and is chiefly valuable from the fact that it forms a black preci- 

 pitate with the salts of the peroxide of iron, and that it forms a 

 tough insoluble compound with gelatine, a form of albumen, and 

 with gelatinous membranes. The first characteristic makes it 

 useful in the manufacture of inks, and the second in the prepara- 

 tion of all kinds of leather. 



In the process of tanning the skins are first soaked for three 

 or four weeks in strong lime water which removes the fat and 

 loosens the hair. They are then washed and treated with very 

 dilute sulphuric acid which removes any remaining lime and 

 opens the pores of the skin fitting it better for the action of the 

 tanning solution. They are then soaked in tannic acid solution 



