136 ANSWERS TO PRACTICAL QUESTIONS 



21. Why do drops of tea stain a knife-blade ? 



(See Chemistry, page 211.) 



The tannic acid of the tea combines with the iron, 

 forming an iron tannate.* 



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



(See Miller's Organic Chemistry, page 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. 



23. Why does writing-fluid darken on exposure to the 

 air? 



(See Chemistry, page 262.) 



It absorbs O, the iron changing to ferric oxide. 



24. What causes the disagreeable smell of a smoldering 

 wick ? 



A volatile substance, termed acrolein, is produced in 

 the decomposition of the oil. 



25. Why does ink corrode steel pens ? 



The free sulphuric acid of the ink combines with the 

 iron of the pen. 



26. How does a bird obtain the CaCO 3 /0r its eggshells ? 



(See chemistry of a hen's egg in Fireside Science.) 



A common hen's egg is 95 per cent, carbonate of lime, 



* 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 tannin acid of tea. 

 softens its flavor. This is generally preferred to the harsh, clear beverage. 



