548 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



which forty-two grains of ammonia of specific gravity 0.960 

 are to be added, and then forty-five grains of absohite al- 

 cohol. 21 A.Decemher, 1872, 1133. 



IMPllOYED LIQUID FOR ETCHING COPPER. 



According to Erckmann, the etching of copper and silver 

 by means of nitric acid has the inconvenience of disensrao-ing- 

 nitrous vapors. This, in addition to the injury to the health 

 and comfort of the operators, often penetrates under the pro- 

 tecting layer of wax or parafiine, and thus produces an unde- 

 sired action. He therefore recommends the use of chromic 

 acid as not being liable to this inconvenience. The action, 

 indeed, is much slower, but the engraver is not rendered un- 

 comfortable, and the engraving is much more clear and sharp. 

 Gold and platinum are not thus acted upon by the acid, but 

 silver becomes covered with a red chromate of silver, which 

 shows that it is attacked. To prepare the chromic acid, 150 

 parts of bichromate of potash are dissolved in 800 parts of 

 warm water, and 200 parts of sulphuric acid are added. This 

 forms a solution which may be used for the j^urpose in ques- 

 tion. 9 B, 1872, XLiiL, 552. 



FORMATION OF ANTIQUE PATINA ON BRONZES. 



A commission appointed in 1864, at the suggestion oftlie 

 late Professor Magnus, has been investigating the conditions 

 favorable to the formation of patina (or green incrustation) 

 on bronzes, in Berlin and its vicinity. Analysis of a collection 

 of antique bronzes showed that while the composition of the 

 bronze does not afifect the beauty of the patina, it may influ- 

 ence the rapidity of its formation, although the latter is also 

 dependent on atmospheric conditions. More recent analyses 

 of patina, as well as of bronze, reveal the presence of earths, 

 etc., in the former which are not found in the latter; a dif- 

 ference due, in the judgment of the committee, to the accu- 

 mulation of dust, etc., rather than to artificial treatment of the 

 antique, as has been suggested. A modern bust, simply ex- 

 posed in the air for seven years, was covered with dust and 

 vegetation, preventing the recognition of the metal beneath ; 

 while another, which had been daily sprinkled w4th water, and 

 gently rubbed monthly with olive-oil, acquired a patina, no- 

 ticeable for its beautiful dark greenish-brown tint, as well as 



