The black stuff?

Question
When I buy a pint of Guinness there is no doubt the liquid is black. Yet the bubbles that settle on top, which are made of the same stuff, are white. The same is true of many types of beer. Why?
Stewart Brown , Bristol, UK

Answer
In the interests of science I poured myself a Guinness and waited until the rising bubbles had formed a creamy head. I put a little of this in a dish and examined it through a low-powered microscope. Unlike bath foam, which has many semi-coalesced bubbles, Guinness foam is made mainly of uniformly sized, spherical bubbles of about 0.1 to 0.2 millimetres in diameter, suspended in the good fluid itself.
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  thanks to reenhead