204 SCIENTIFIC AMUSEMENTS. 



regia, from its power of dissolving the royal metal. The solution 

 takes place because the two acids react on one another in such a 

 fashion as to develop chlorine, Avhich combines with the gold and 

 is the real agent in forming a soluble substance. 



A locket, ring, chain, &c., may be tested to see if it is made of 

 gold or not, by rubbing the article on a tile or stone so as to form 

 a streak of yellow metal like a pencil mark ; if this disappears 

 instantly on moistening with a drop of nitric acid the metal is not 

 gold ; but if the acid has little or no action on the streak, the 

 metal is gold of tolerably good quality.* 



Expt. 231. To obtain a bright light from Magnesium. The 

 metal magnesium is sold in the form of wires and flattened ribbons 

 for the purpose of burning so as to produce a bright light. Hold a 

 bit of magnesium wire in the flame of a Bunsen lamp for a few 

 moments ; the wire will soon take fire and burn with a most 

 brilliant light, a light white substance termed mar/nesia or 

 magnesium oxide being produced instead of the solid wire. This 

 arises from the circumstance that the magnesium (when hot 

 enough) absorbs oxygen from the air and a vigorous chemical 

 action is set up, the heat of which keeps the magnesium active as 

 long as air is supplied (just as a candle keeps burning under 

 similar conditions). 



Expt. 232. To burn Iron and Zinc in Air. Into the flame of a 

 Bunsen burner or spirit-lamp sprinkle iron or zinc filings (best from 

 a pepper-castor); brilliant sparkles will be produced owing to the 

 absorption of oxygen from the hot air by the metal and the 

 chemical action (oxidation) set up, just as in the case of magnesium. 

 Iron or steel or thin ribbons or wires can be readily burnt in 

 oxygen gas although not so easily in air ; zinc when rolled out into 

 very thin sheets can be burnt in a candle flame almost as easily as 

 paper. 



Many other metals besides magnesium, iron, and zinc behave in 

 a similar fashion; when heated sufficiently in contact with air 



* The quality of gold is usually expressed by imagining the whole of the 

 material to be divided into twenty-four parts called carats, and mentioning 

 the number of these parts which are pure gold, the balance being alloy of 

 various kinds added either to harden and render capable of resisting wear 

 and tear, or to cheapen. The standard for British gold coins or "guinea- 

 gold " is 22 carats ; i.e., ff = H of the whole is pure gold, and T V other metal 

 added to harden ; in Australian sovereigns a little of this T V is silver and the 

 rest copper, whence the yellower shade ; in ordinary coins the whole of the T V 

 is copper. French gold coin contains > of its weight of true gold and T \ of 

 alloy, so that English and French coin have not quite the same composition, 

 the former containing 91 1 in 100 of gold, and the latter only 90 in 100. 

 15 carat gold contains 44 or f of its weight of precious metal ; 9 carat gold 

 only o-\ or f , and so on. 



