Today In Science History – June 17 – Crookes and Thallium

William Crookes
William Crookes in 1875 (age 43). Credit: Popular Science Monthly Volume 10, 1876.

June 17 is William Crookes birthday. Crookes was an English chemist best known for the discovery of the element thallium.

Crookes was sent a ten pound sample of sludge from his former teacher August von Hofmann. Crookes had moved his research from organic chemistry to the study of compounds of selenium. The sludge was a byproduct from a sulfuric acid factory in Tilkerode, Germany and contained some selenium. Hofmann wanted a method of extracting the selenium from the sample. Crookes performed a spectral analysis on the sample and confirmed the sludge contained a lot of selenium. He also spotted a bright green line he had never seen before. Further investigation proved the green line belonged to a new element. He named the element thallium after the Greek word thallos meaning budding twig or new growth after the green line that led to the discovery.

Crookes Radiometer
Crookes radiometer in action. Note the dark sides moving away from the light. Credit: ©Nevit Dilmen/Creative Commons

Crookes was also known for the Crookes radiometer, which is sold in many museum gift shops and novelty stores. Crookes invented the device after trying to weigh out chemical samples on a sensitive, partially evacuated scale. He noticed the scale would move slightly when struck by sunlight. He built the radiometer to investigate this phenomenon. The device consists of an evacuated glass bulb containing a low-friction spindle with three or four lightweight metal vanes. The vanes are shiny on one side and blackened on the other. When light is shined upon the vanes, the spindle will begin to turn due to the light pressure. The shiny sides move towards the light and the dark sides are pushed away. If the radiometer is cooled, it will spin in the opposite direction. Later it was discovered infrared light had the most effect on the device. It could even begin to turn in the dark if you placed your hands around the bulb. The heat from your hand would produce enough infrared radiation to turn the vanes. Today it is used in classrooms to demonstrate the principles of a heat engine powered by light. That, and it makes a nifty desk ornament.

Happy birthday William Crookes!