568 History of Luminescence 



decomposition had set in, although it was weaker. To his astonish- 

 ment, a medusae that had remained on a sheet of paper for twenty- 

 two hours and which had mostly liquefied and was completely dark, 

 became very bright on adding fresh water (but not sea water) , 

 bright enough to read print. Spallanzani referred to these liquefied 

 medusae as dried medusae and has been credited with observing that 

 luminous animals could be dried and would again luminesce on 

 moistening, but it is a question as to how dry they really were, espe- 

 cially since he found that movement, as by rubbing the " dried " 

 dark medusae with his finger would revive the light again. He wrote: 



Another medusa which was dead, and had not been luminous for 

 some time, was lying, out of the water, in the window of my chamber 

 during the night. A slight rain chanced to fall, and every drop which 

 fell on the dead medusa was changed into a brilliant spangle, till in a 

 short time the medusa was studded all over with such shining points. I 

 could produce no such effect by sprinkling it with sea-water in imitation 

 of rain. 



By crushing two large medusae in thirteen ounces of water until no 

 more light appeared, an " artificial phosphor " could be made which 

 gave light when heated to 30° Reaumur (100° F.) . 



Spallanzani found that the luminous material was a mucous 

 formed on the edge of the umbrella and especially abundant on the 

 arms. The mucous luminesced on mixing with water, urine, or 

 milk. The rest of the animal, devoid of this mucous, or the " sap " 

 of body tissue, could not be made to light in any way. The lumi- 

 nescence in milk was so great " that I could read the writing of a 

 letter at three feet distance." 



It was early recognized that the light of medusae only appeared 

 as a result of mechanical stimulation. Alexander von Humboldt 

 (1769-1859) was apparently the first to discover that luminescence 

 could be elicited by electrical stimulation. In his Travels to the 

 Equinoctial Regions of the New Continent during the years 1799- 

 1804, medusae were described as abundant between Madeira and 

 Teneriffe in the Canaries. He wrote,^^ 



If we place a very irritable medusa on a pewter plate, and strike against 

 the plate with any sort of metal, the small vibrations of the plate are 

 sufficient to make this animal emit light. Sometimes in galvanizing the 

 medusa, the phosphorescence appeared at the moment the chain closes, 

 though the exciters are not in immediate contact with the organs of the 

 animal. 



There is no detail regarding the electrical apparatus, but it was no 



** From the English edition, 1853. 



