^jn™!i£rm'\      JVhat  the  Atmosphere  is  Made  of.  257 
and  having  no  other  notion.  Mayow  found  out  that  air  has 
"  nitro-aereal  particles  "  and  "  mephitic,"  and  Hales  discovered  that 
a  great  deal  of  gas  can  be  derived  from  small  quantities  of  solids. 
He  distilled  many  things  in  his  laboratory,  including  what  are 
modern  products ;  he  saw  that  air  is  a  chaos  of  elastic  and  inelastic 
things,  but  he  failed  to  catch  the  import  of  his  discoveries.  Thus 
far  everything  had  been  hypothetical,  there  was  no  standard  used 
and  phlogiston  was  ready  at  every  turn  to  account  for  strange 
results. 
Joseph  Black  was  the  first  real  discoverer;  he  brought  into  re- 
quisition delicate  scales,  and  came  upon  the  idea  of  changed  chemi- 
cal condition  later  developed  into  "  conservation  of  matter."  An 
experiment  with  magnesia  was  repeated  by  the  lecturer  in  much  the 
same  form  as  worked  by  Black,  in  which  after  decomposition,  the 
magnesia  was  reconstructed.  Black's  pupil,  Rutherford,  born  in 
1749,  took  for  his  thesis  the  quantities  of  a  cold,  fixed  air,  which 
was  really  an  investigation  into  the  nature  of  the  residual  gases 
after  certain  kinds  of  experiment. 
It  was  Priestley,  born  in  1733,  who  discovered  oxygen,  and  he 
had  such  confidence  in  its  life-giving  qualities  that  he  inhaled  it, 
looking  forward  to  the  time  when  others  might  avail  themselves 
of  the  luxury,  "  which  till  now  has  been  enjoyed  only  by  two  mice 
and  myself."  Then  came  Lavoisier,  who  first  mentions  nitrogen 
under  the  name  it  still  has  with  the  French,  azote,  he  determining 
that  there  are  four  kinds  of  air,  common  air,  pure  air,  which  is 
indeed  oxygen,  azotic  gas  and  fixed  air,  which  is  carbonic  acid 
gas.  He  had  really  the  mystery  of  the  air  within  his  grasp,  and 
produced  hydrogen  from  water,  but  in  the  very  last  of  his  many 
memoirs  defended  phlogiston. 
The  general  historical  story  ended  with  Cavendish,  quiet  and 
retiring,  almost  unknown  save  in  very  limited  circles,  who  dis- 
covered the  composition  of  water.  He  maintained  the  idea  of 
phlogiston  although  the  lecturer  showed  that  he  did  so  from  the 
desire  to  speak  to  his  contemporaries  in  current  phraseology  rather 
than  on  account  of  belief  in  it.  He  did  question  to  himself  the 
validity  of  this  curious  substance  and,  had  he  used  the  chemical 
balance  in  his  experiments,  would  undoubtedly  have  discarded  it 
altogether.  He  plainly  states  that  where  there  is  phlogiston  there 
is  always  water  and  suggests  that  it  may  be  the  water  that  is  ef- 
fective.   He.  worked  largely  for  his  own  pleasure  and  was  not 
