They are probably not metallic, but are grit that has washed off the roofing shingles.
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Acid rain contain carbonic acid, sulfuric acid, nitric acid.Acid rains are very dangerous for metallic structures and plants.
air Apex :)
Rain water does contain salts but not those which are found in sea water. Rain contains salts which are usually obtained from atmospheric pollutants such as nitrates, sulphates, and so on.
The correct answer is Oxygen, otherwise air. (APEX)
you can find this information out at the following places I found for you to look at www.thermawire.com/ ,www.warmfloor.com/ . I hope these sites can help you out with this stuff.