494- W>t Igms [Book V. 



noife in the large meteors only takes place when the 

 mafs feparates or goes off like a fky-rocket, and in 

 this cafe the effecl: is fimilar to that of gunpowder, or 

 any difploding matter. 



Concerning the nature and compofition of the ignis 

 fatuus, or will-o'-the wifp, there is lefs difpute ; the 

 generality of philofophers being agreed that it is caufed 

 by fome volatile vapour of the phofphoric kind, pro- 

 bably the phofphoric hydrftgen gas. The light from 

 putrefcenc fubilances *, particularly putrid fifh, and 

 thofe fparks emitted from the fea, or fea-water when 

 agitated in the dark, correfpond in appearance with this 

 meteor. Sir Ifaac Newton defines the ignis fatuus to 

 be " a vapour Ihining without heat ;" and it is ufually 

 vifible in damp places, about dunghills, burying 

 grounds, and other fituations, which are likely to 

 abound with phofphoric matter. 



A remarkable ignis fatuus was obferved by Mr. 

 Derham, in fome boggy ground, between two rocky 

 hills. He was fo fortunate as to be able to approach 

 it within two or three yards. It moved with a brifk 

 and defultory motion about a dead thiltle, till a flight 

 agitation of the air, occafioned, as he fuppofed, by his 

 near approach to it, caufed it to jump to another place j 

 "and as he approached, it kept flying before him. 

 He was near enough to fatisfy himfelf, that it could 

 not be the mining of glow-worms or other infects 

 it was one uniform body of light. 



M. Beccaria mentions two of thefe luminous ap- 

 pearances, which were frequently obferved in the 

 neighbourhood of Bologna, and which emitted a light 

 equal to that of an ordinary faggot. Their motions 

 were unequal, fometimes rifmg, and fometimes fink- 

 ing towards the earth; fometimes totally difappearing, 



* This fubjeft will be more amply treated of in the fucceeding 

 !?ook, under the title Phofphorus, Book VIII. 



though 



