140 History of Luminescence 



We must own therefore, that the Motion of the Parts which we suppose 

 in luminous Bodies, is to be found here also. 



It is not so easy to tell certainly, what sort of Motion that is, which 

 makes some Worms and Flies to shine in the Dark: However it is very 

 probable, that some sort of Matter is exhaled out of these Insects, like 

 the Sweat of other Animals, and that this pushes the Matter of the 

 second Element; and this is confirmed from hence, that they cease to 

 shine as soon as they are dead. 



The luminosity of precious stones gave some difficulty because it 

 was not easy to visualize the particles of such hard bodies in motion 

 and Rohault was inclined to doubt that they emitted light. He 

 wrote: 



If it were true, what they say of a Carbuncle and a Diamond, viz. that 

 they shine in the Dark; I should freely own, that I am mistaken in all 

 that I have said about Light; for there is no Probability, that Bodies so 

 hard, should be composed of Parts which separately are in any Sort of 

 Agitation. But it is certain, that these are only idle Stories, told without 

 any Proof, and received by credulous Persons, for I have often times 

 experienced the contrary my self. 



'Tis true indeed, that a Diamond shines very bright in a darkish Place; 

 but the Reason of this is, because it is so cut that the Sides reflect all 

 the Light which they receive towards the same Part, as shall be more 

 fully explained afterwards, when we come to treat of the Refraction of 

 Light. 



We have lately had an Account from England,^^ that some Diamonds 

 rubbed in the Dark, have shined so bright for a short time, that a Word 

 or two might be read by Light of them. I have not observed this in 

 any Diamonds that I have tried; however it may be true, without con- 

 tradicting any Thing that I have hitherto wrote. For the Rubbing may 

 raise some Agitation, if not in the Parts of the Diamond, yet at least in 

 some Matter contained in the Pores of it, which continuing in Motion 

 in the same manner as the Flame in the Pores of a burning Coal, may 

 for some time push the second Element which is all round it, and dis- 

 pose it to raise a small Sensation of Light. 



After explaining how the Bononian stone imbibes light and then 

 emits it in the dark (see Chap. VHI) , Rohault discussed the proper- 

 ties of salt, its abundance in the sea, the flow and ebb of the tide, 

 and other matters pertaining to the ocean. He had no conception 

 of the animal origin of the light of the sea, but, like Descartes, 

 attributed the sparks seen on violent agitation of sea water to par- 

 ticles of salt. These salt particles become disengaged from the water 

 and " dart themselves into the Air with their Points forward " and 

 thus communicate a force " sufficient to impel the second Element, 



8* The work of Robert Boyle. 



