XI. A, 4 



Argiielles: Galvanized-iron Roofing 



181 



ities. On account of the presence of adhering dirt and films 

 of zinc oxide and carbonate, the zinc was determined directly 

 by precipitation as carbonate, ignition, and weighing as oxide. 

 The results obtained are as follows : 



Table II. — Amount of zinc on old galvanized sheets/ 



[Flat sheet ; all 22 gauge.] 



Tracing 

 No. 



No. of 

 sample. 



Year 

 placed 

 under 

 service 

 expo- 

 sure. 



Weight of coating 

 per— 



Square 

 decime- 

 ter. 



Square 

 foot. 









Grams. 



Ounces. 



1 



1 



1883 



7.15 



2.34 



2 



1-X 



1883 



8.33 



2.72 



3 



2 



1882 



8.96 



2.93 



4 



2-X 



1882 



11.02 



3.60 



5 



3 



1885 



9.71 



3.18 



6 



3-X 



1885 



9.98 



3.26 



7 



4 



1882 



8.21 



2.68 



8 



4-X 



1882 



8.69 



2.89 



9 



5 



1885 



7.33 



2.40 



10 



5-X 



1885 



8.42 



2.76 



11 



6 



1886 



9.07 



2.97 



12 



6-X 



1885 



12.63 



4.13 



13 



7 



1883 



7.23 



2.36 



14 



8 



1882 



8.89 



2.91 



15 



8-X 



1882 



10.15 



3.32 



16 



9 



1884 



8.78 



2.88 



17 

 18 



blO 

 bll 





9.71 

 10.47 



3.18 

 3.43 



1880 



" Samples marked X were taken from the unexposed lap of the sheets- 

 " Old sheets not continuously under sei-vice exposure. 



For the determination of the weight of coating per unit area 

 on modern, unused sheets, the following procedure was adopted : 



Pieces of galvanized sheets cut accurately to 7.62 by 10.16 

 centimeters (3 by 4 inches) or 5.08 by 5.08 centimeters (2 by 

 2 inches) are cleaned, washed with alcohol and ether, and are 

 dried and weighed, after which they are immersed in a sulphuric 

 acid solution " (containing 3 per cent of the acid by weight) 

 to remove the zinc. The complete solution of zinc is marked by 

 the cessation of hydrogen evolution. By this process the iron 

 base is attacked very little. After the reaction is completed, 

 the iron is brushed, washed, dried, and weighed. The difference 

 between the original weight and the weight after immersion 

 in acid represents the zinc plus a small amount of iron and any 



" Burgess, loc. cit., recommends 0.G6 N, or 3.2 per cent sulphuric acid. 



