PECULIAR FOG IN ICELAND. 155 



are reversed and less vivid ; but as soon as I removed the 

 jet they both disappeared." 



"XII. The colours came from the reflection and re- 

 fraction of the sun's rays, in a certain manner, in the 

 drops of water ; it follows that this must take place dif- 

 ferently in vapours and in drops of water not hollow. 

 Now there can be no other cause of this difference than 

 the interior figure, because the exterior is the same. 

 Therefore the vapours are hollow vesicles." 



He then found that, in a globe with water, when the air 

 was compressed he saw no vapour till he made an opening 

 to the air, when the vapour became visible and the colours 

 of the rainbow appeared. He also formed visible vapours 

 by pumping the air out of a globe with aqueous vapour. 

 Some particles he found different from others, and twelve 

 times as fine as a hair; the diameter of the hair was 

 the three-hundredth part of an inch, and of the vapour 

 particles 3^. 



He concludes, from the experiment by exhaustion, that 

 the vapours which keep themselves in the air naturally are 

 invisible, and become visible when they descend and expand. 



He considers that the rays are refracted from the 

 vapours as from soap-bubbles. To the objection that the 

 refraction may be in the concrete bubble, he replies that 

 "some vapours show an order of colours very different 

 from others; vapours are, therefore, of different sizes. 

 They do not reflect colour from their whole volume." 



To reflect colours in the order seen, he says the pel- 

 licle of the vapours must be very fine. Then he wishes to 

 show that air is necessary to keep up vapours, because if 

 the air is pumped out the vapours become visible. 



He thinks the elastic fluid causing ebullition in a 

 vacuum goes through the pores of the vessel. 



He sees that vapours have no absolute lightness, and 

 cannot lose their specific gravity. 



