STEVE LOWRY / SCIENCE PHOTO LIBRARY STEVE LOWRY / SCIENCE PHOTO LIBRARY
Polarised light micrograph of murexide, also known as ammonium purpurate or Roman purple. A common test for the presence of acid in urine depends upon the formation of murexide (an ammonium salt), which is an intense reddish purple. It was formerly used in dyeing calico, and was obtained in large quantities from guano. Magnification: x270 at 10cm wide.
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