PKACTICAL  THOUGHTS  ON  LIGHT.  75 
PRACTICAL  THOUGHTS  ON  LIGHT. 
By  James  S.  T.  W.  Smith. 
Some  time  ago,  while  sitting  at  my  desk,  I  observed  that  one 
of  the  show  carboys  in  the  window  had  focussed  the  sunlight 
on  my  coat-sleeve,  and  was  burning  it.  It  is  quite  possible  that 
many  fires  may  have  taken  place  in  this  way  when  no  satisfac- 
tory reason  could  be  given  for  their  occurrence.  It  would  be 
well  in  arranging  goods  and  show-glasses  in  druggists'  shops, 
to  bear  in  mind  this  fact,  and  to  so  place  goods  (especially  any 
which  would  readily  take  fire)  that  focussed  rays  of  light  would 
never  fall  on  them. 
The  circumstance  caused  me  to  reflect  upon  the  general  in- 
fluence which  light  exerts  upon  pharmaceutical  preparations 
stored  in  shops ;  and  knowing  the  prejudicial  effects  of  light 
upon  chemicals,  it  is  very  natural  to  ask  if  it  cannot  to  a  great 
extent  be  prevented  from  doing  so  much  injury. 
In  order  that  the  subject  may  be  made  as  plain  as  possible, 
it  may  not  be  amiss  to  give  a  general  idea  of  the  nature  of 
light,  and  how  it  acts.  To  be  as  brief  as  possible,  it  may  be 
remarked  that  a  ray  of  light  has  been  found  to  have  three  dis- 
tinct properties  at  the  least ;  namely,  the  property  of  giving 
heat  and  light,  and  the  power  of  producing  chemical  action. 
Men  of  science  have  demonstrated  this  for  us  by  passing  a 
ray  of  light  through  a  prism,  and  then  examining  the  spectrum 
produced.  The  spectrum,  as  first  produced  by  Newton,  ap- 
peared to  him  to  consist  of  seven  primitive  colors  ;  but  since  his 
time  others  have  represented  that  there  are  only  three,  and  that 
the  seven  are  produced  by  the  mixing  of  the  edges  of  the 
colored  bands  in  the  spectrum.  Thus  assuming  blue,  red,  and 
yellow  to  be  the  primitive  or  fundamental  colors,  the  other  colors 
of  the  spectrum  are  represented  to  be  produced  by  the  meeting  and 
overlapping  of  the  bands,  blue  and  yellow,  for  instance,  producing 
green.  It  has  been  found  that  the  red  ray  contains  the  most 
heat,  the  yellow  ray  the  most  light,  and  the  blue,  or  violet  ray, 
exercises  the  greatest  amount  of  chemical  action.  But  while 
the  heating  effect  of  the  ray  of  light  extends  beyond  the  visible 
part  of  the  spectrum  at  its  red  extremity,  so  also  the  chemical 
effect,  or  actinism  as  it  is  called,  extends  beyond  the  visible 
