ON  NASCENT  MANURES. 
247 
substance  and  in  the  same  solution,  will  grow  and  become  more 
perfect,  at  the  expense  of  those  which  are  irregular.  Upon  this 
principle,  the  imperfect  crystals  may  be  said  to  be  approximating 
to  the  allontropic  condition,  or  nascent  state,  while  the  perfect 
crystal  is  in  the  normal  condition. 
It  may  be  said  that  extent  of  surface  is  one  of  the  causes  of 
this,  and  a  better  illustration  is  sand,  or  quartz,  which  is  perfectly 
insoluble  in  its  natural  or  normal  condition,  however  fine  the 
powder,  even  in  some  of  the  strongest  acids.  But  sand  or  silica 
is  frequently  found  in  the  nascent  condition,  and  then  it  dissolves 
readily  in  water;  moreover,  it  can  be  kept  in  this  condition  for 
years;  but  if  heated  to  the  temperature* of  260°  it  assumes  its 
normal  condition,  and  becomes  perfectly  insoluble  even  in  acids; 
whereas  before,  it  would  dissolve  in  acids,  alkalies,  or  pure 
water. 
Lime  and  Magnesia,  while  in  the  caustic  state,  are  capable  of 
converting  sand  into  soluble  silica  ;  and  this  is  perhaps  one  of  the 
good  effects  of  liming,  especially  when  we  consider  the  remarka- 
ble influence  that  soluble  silica  exerts  in  absorbing  ammonia  from 
the  atmosphere,  and  also  from  ammoniacal  manures.  We  may 
also  account  thus  for  the  crumbling  of  stable  walls,  the  moist  con- 
dition of  old  walls,  and  especially  those  that  are  exposed  to  am- 
moniacal exhalations.  Moreover,  we  have  a  plausible  mode  of 
accounting  for  nitre  beds,  and  the  remarkable  value  of  old  plas- 
ter ;  also  the  purifying  influence  of  "  White-washing,"  if  it  is 
done  with  caustic  lime,  and  not  with  whiting  or  carbonate  of  lime. 
Lime,  while  caustic  and  moist,  in  contact  with  sand,  converts  a 
small  part  of  the  surface  of  the  grains  of  sand  from  the  soluble 
to  the  insoluble  silica  ;  and  this  is  the  reason  why  caustic  lime  is 
necessary  to  the  formation  of  good  mortar,  as  it  is  not  (as  is  al- 
*  most  universally  supposed)  a  mere  mechanical  mixture  of  limeand 
sand,  neither  is  it  grains  of  sand  cemented  together  by  the  indura- 
tion of  lime,  but  the  actual  solution  of  the  surface  of  the  grains 
of  sand  produces  a  still  more  intimate  union. 
Well,  this  soluble  silica  gradually  absorbs  from  the  atmosphere 
the  ammonia,  for  which  it  has  a  remarkable  affinity  ;  and  as 
ammonia  is  the  vehicle  of  poisonous  exhalations  of  disease,  as 
well  as  the  perfume  of  flowers,  these  exhalations  are  so  concentra- 
ted upon  the  walls  of  hospitals,  that  it  sometimes  becomes  neces- 
