189 
observed at the bottoms of the beakers even after only 12 
hours’ exposure) ; but, after a time, this green compound 
apparently ceased to form, giving place to a thin but not 
easily removable covering of a bronze colour on the surface 
of the foil, on which the water ceased to act ; on removing 
this tarnished foil, however, to some fresh sea-water, action 
again commenced, resulting as before in the formation of a 
green-coloured deposit. So as to find whether the bronze 
covering given to the copper by the sea-water prevented or 
not the action of the waters upon the copper, one of the 
pieces of tarnished foil was immersed in 2,000 grain measures 
of soft river water, and into a similar quantity of the same 
water a piece of bright copper-foil of the same size was 
placed. After the expiration of five days the water was 
examined. It had acted considerably upon the bright cop- 
per, but only slightly on the tarnished, — the copper dissolved 
in one case being about half as much as in the other. The 
tarnished surface, then, does not prevent totally the action 
of soft water upon the copper ; or the tarnish is itself acted 
upon. 
Before testing, in each case the green compound was 
separated from the water, so that the amount of copper 
might be determined in each separately. The following 
results were obtained : — 
(a) Copper in solution in the water. 
Days Exposed. Copper. 
4 0*00675 of a grain. 
7 0*00350 
14 0*00300 
32 0*00300 
Here we have an instance of the copper dissolved reach- 
ing its maximum, then between the 4th day and the 7th 
day diminishing from 0*00675 to 0*00350; but, as will be 
seen below, the green compound continued to form. 
