GASES ARE ALWAYS TRANSPARENT. 71 



kind of condition as regards heat that I did the 

 water and the ether [putting a few grains of 

 iodine into a hot glass globe, which immediately 

 became filled with the violet vapour], and you 

 see the same kind of change produced. More- 

 over, it gives us the opportunity of observing 

 how beautiful is the violet-coloured vapour from 

 this black substance, or rather the mixture of 

 the vapour with air (for I would not wish you 

 to understand that this globe is entirely filled 

 with the vapour of iodine). 



If I had taken mercury and converted it into 

 vapour (as I could easily do), I should have a 

 perfectly colourless vapour, for you must under- 

 stand this about vapours, that bodies in what 

 we call the vaporous, or the gaseous state, are 

 always perfectly transparent, never cloudy or 

 smoky ; they are, however, often coloured, and 

 we can frequently have coloured vapours or 

 gases produced by colourless particles themselves 

 mixing together, as in this case [the Lecturer 

 here inverted a glass cylinder full of binoxide of 

 nitrogen ( 1G ) over a cylinder of oxygen, when the 

 dark red vapour of hyponitrous acid was pro- 

 duced]. Here also you see a very excellent 

 illustration of the effect of a power of nature 



F 4 



