186 ECONOMIC GEOLOGY 



Tin unites with mercury in the formation of an amalgam used 

 extensively in the filling of teeth, and when amalgamating anew 

 with silver as a cement in joining the teeth together. Tin does 

 not readily alloy with zinc alone. 



Tin is used also in the detection of gold. A solution of stannous 

 chloride in the presence of stannic chloride or free chlorine is one 

 of the most delicate tests known for the yellow metal. The com- 

 pound formed whose composition has been somewhat of an open 

 question is known as the purple of Cassius. 



An interesting point appears concerning the effect of tin on 

 canned goods. According to F. Wirthle, who examined a large 

 number of samples of canned beef, the white crust sometimes 

 formed within the can was a basic tin chloride resulting from the 

 action of the salt on the tin surface either by the direct action of 

 the tin or by freeing first some organic salts of the tin, which was 

 subsequently converted into a basic sodium stannous chloride and 

 some organic sodium salt. From a large number of determina- 

 tions the maximum of tin obtained was 0.014 per cent, in a can 

 of beef that was five years old. The interior of the can was cor- 

 roded in all cases where tin was in contact with fat, and not in a 

 single instance had corrosion begun where tin was in contact with 

 gelatin. The test employed in the detection of the tin was a 

 solution of stannous chloride and ammonium molybdate. The 

 test is delicate to 21-5,000,000 part of tin in solution. Even with 

 this extreme dilution a beautiful blue coloration is observed. 



There have been three sudden advances in the production and 

 consumption of tin. The first came in the fourteenth century, 

 which was marked by the use of tin for bell metal. The second 

 came in the eighteenth century which was marked by the large 

 use of tin in bronze for artillery. The third comes in the twen- 

 tieth century which is marked by a large consumption of tin in 

 canned provisions. 



Tin is also utilized in the manufacture of tin foil for wrapping 

 many small articles of commerce. The foil consists of a very 

 thin sheet of lead coated with a thin film of tin. Tin is also used 

 in silvering mirrors when amalgamated with mercury. The chlo- 

 ride of tin is used in dying and printing. The artificial oxide 

 of tin under the name of putty powder is used in polishing 

 granite. The polish thus obtained is far more lasting than that 

 produced by oxalic acid. 



During the last few years the separation of tin from tin plates, 



