Different Kinds of Coal for the purpose of Illumination. 277 



I 



Before concluding I wish to advert briefly to a mode of fix- 

 ing the illuminating power, which, so far as 1 am aware, has 

 not been publicly noticed in any of the papers lately printed. 

 Dr Lyon Playfair, who described it to Mr King of the Gas- 

 Works, Liverpool, from whom I first got the account of 

 it, ascribes it to Professor Bunsen of Marburg; and, accord- 

 ingly, as I understand, he has the merit of proposing it. It 

 consists of a sheet of paper, besmeared with spermaceti, ex- 

 cepting at a small part at the centre, by which the besmeai-ed 

 portion becomes more pervious than the other to light; and 

 consequently a light placed behind it, causes adark spot on that 

 l)art not covered. When another light is placed before 

 the paper, the spot is distinctly visible, if that light be 

 placed at such a distance as to cause the reflexion from 

 the paper to be either of greater or of less intensity than 



