33 o SCIENCE PROGRESS 



not when it is cold but because when hot it contains sulphuric 

 anhydride, which acts as the depolanser — 



Cu + S0 4 H., + OS0 3 = CuSO, + H,0 + S0 2 . 



The fact that copper is soluble even in very weak acid 

 solutions in presence of oxygen has long been obvious to all 

 users of copper vessels. In every well-conducted kitchen, 

 copper vessels are always kept scrupulously clean and dry; 

 no properly trained cook ever allows food materials to stand 

 any length of time in a copper pan. In recent years, from 

 motives of cleanliness, brewers have been led to line their 

 fermenting vats with copper and have been somewhat surprised 

 to find that fermentations took place in a less satisfactory 

 manner than in wooden vessels and that the growth of the 

 yeast was appreciably diminished. The change has been traced 

 to the presence of small quantities of copper in solution. 

 Apparently, action takes place at the air-water line, the copper 

 being dissolved in a circuit formed between the metal, atmo- 

 spheric oxygen and the minute proportion of acid present in 

 the wort. Brewers are being led to use aluminium in place 

 of copper now that it is recognised that the copper is liable 

 to dissolve in the cold wort. 



Copper can be used safely in the kitchen and in constructing 

 the huge vessels used in breweries in boiling the wort before 

 it is fermented, because the amount of oxygen in solution in 

 water is so very much reduced by heating it. 



Attention has already been called (p. 314) to another pro- 

 blem awaiting solution in the case of copper, namely, whether 

 pure copper will dissolve in acid in presence of an effective 

 depolariser. Copper, as ordinarily prepared, probably contains 

 sufficient electro-negative impurity to condition its dissolution, 

 if such be required. 



A knowledge of the conditions which determine the corrosion 

 of copper alloys, especially of the copper-zinc alloys, is of 

 importance, owing to the large use that is made of these alloys 

 in engineering practice. 



Attention has been called to the subject of late, as it has 

 been under discussion at meetings of the recently established 

 Institute of Metals. A long Report to the Corrosion Committee 



