LUMINOUS INSECTS. 



345 



by immersion in water, or increased by the application of 

 heat ; that the substance affording it, though poetically em- 

 ployed for lighting the fairies' tapers is incapable of in- 

 flammation if applied to the flame of a candle or red-hot iron ; 

 and when separated from the body exhibits no sensible heat 

 on the thermometer's being applied to it — rejects the pre- 

 ceding hypotheses as unsatisfactory, but without substituting 

 any other explanation; suggesting, however, that the facts 

 he observed are more favourable to the supposition of light 

 being a quality of matter than a substance. 2 Lastly, Dr. 

 Todd finding that the luminous substance of Lampyris con- 

 tinues to shine when detached, sometimes for a longer and 

 at others a shorter period, but never exceeding twenty 

 minutes, and that under mercury, various gasses, water, and 

 in vacuo, considers it solely as an effect of vitality. 3 



Which of these opinions is the more correct I do not pre- 

 tend to decide. But though the experiments of Mr. Ma- 

 cartney seem fairly to bear him out in denying the existence 

 of any ordinary combination of phosphorus in luminous 

 insects, there exists a contradiction in many of the state- 

 ments, which requires reconciling before final decision can 

 be pronounced. The different results obtained by Forster 

 and Spallanzani, who assert that glow-worms shine more 

 brilliantly in oxygen gas, and by Beckerheim, Dr. Hulme, 

 and Sir H. Davy, who could perceive no such effect, may 

 perhaps be accounted for by the supposition that in the 

 latter instances the insects having been taken more recently, 



1 "And for night- tapers crop their waxen thighs, 



And light them at the fiery glow-worms' eyes." 



2 Some experiments made hy my friend the Rev. R. Sheppard on the glow- 

 worm are worthy of being recorded. — One of the receptacles being extracted 

 with a penknife, continued luminous ; but on being immersed in camphorated 

 spirit of wine, became immediately extinct. The animal, with one of its 

 receptacles uninjured, being plunged into the same spirit, became apparently 

 lifeless in less than a minute ; but the receptacle continued luminous for five 

 minutes, the light gradually disappearing. — Having extracted the luminous 

 matter from the receptacles, in two days they were healed, and filled with lumi- 

 nous matter as before. He found this matter to lose its luminous property, and 

 become dry and glossy like gum, in about two minutes ; but it recovered it 

 again on being moistened with saliva, and again lost it when dried. When the 

 matter was extracted from two or three glow-worms, and covered with liquid 

 gum-arabic, it continued luminous for upwards of a quarter of an hour. 



3 Phil. Trans. 1824. 



