104 ANSWERS TO PRACTICAL QUESTION'S 



1 7. Is the oxalic acid in the pie-plant poisonous ? 



It is neutralized by the alkaline base, with which it is com- 

 bined in the plant. 



18. How may ink-stains be removed ? 



By a solution of oxalic acid, forming an iron oxalate which 

 is soluble in water, and hence may be washed out. 



W. Wliy is leather black on only one side ? 



The solution of copperas, which blackens the leather, is ap- 

 plied on only one side. 



20. JfJiy do drops of tea stain a knife-blade ? 



The tannic acid of the tea combines with the iron, forming 

 an iron tannate.* 



21. Why will not coffee stain it in the same way ? 



(See Miller's Organic Chemistry, p. 549.) 



The modification of tannin contained in coffee, unlike that 

 in tea, turns a solution of ferrous sulphate green, and will not 

 precipitate one of gelatin. 



22. Why does ivriting-fluid darken oti exposure to the 

 air? 



It absorbs O, the iron changing to ferric oxide. 



23. IfJiat causes the disagreeable smell of a smolder- 

 ing ^vick ? 



A volatile substance, termed acrolein, is produced in the de- 

 composition of the oil. 



24. IVJiy does ink corrode steel pens ? 



The free sulphuric acid of the ink combines with the iron 

 of the pen. 



* The tannic acid of the tea tans the albumen of the milk used in 

 seasoning the tea, forming flakes of real leather. It has been calculated 

 that an average tea-drinker, in this way, makes and drinks enough leather 

 each year to make a pair of shoes. The albumen of milk uniting with the 

 tannic acid of tea, softens its flavor. This is generally preferred to the 

 harsh, clear beverage. 



