Algae often invade swimming pools and make for greenish, cloudy water. A very small amount of copper sulfate added to the water improves the clarity and makes the pool more inviting. There is another benefit: the use of copper sulphate will cut down on the amount of chlorine or bromine used to disinfect the pool. Copper sulphate at such dilutions is an extremely safe substance and has been given a clean bill of health by the Pest Management Regulatory Agency of Health Canada. This agency also regulates a completely different use for copper sulphate. It can be used to control "downy mildew fungus" which can devastate grapevines. It frolics in wet weather and first shows up as a downy patch on the bottom of the leaf. If rain persists, it becomes established and can destroy the crop and even carry over into the next season. In the 1860s, Pierre-Marie-Alexis Millardet, a professor of botany at the University of Bordeaux discovered that a mixture of copper sulphate and lime effectively eliminated the fungus. How did he discover this? Farmers had long been spraying vines with the mix to produce unappetizing grapes to deter thieves. Millardet noted that these vines had no fungal growth. The combination of copper sulphate and lime came to be known as Bordeaux mixture and became the first commonly used fungicide. Because it is made from naturally occurring minerals it can be used in organic agriculture. This in spite of the fact that toy companies who make chemistry sets will not include copper sulphate because they judge it to be too dangerous! Growing beautiful blue crystals of copper sulphate used to be a common science activity for students but now if they want to pursue this activity they have to purchase the copper sulphate in a health food store where it can legally be sold as an organic fungicide.