Sii/ip/i’iiif.’nl  It)  Ihe  “ Tropical  A(j)'icaiUiri)<l.." 
[-)uly  1,  1SD5. 
inetliods  and  iiialei'iiils  of  plant  nnurisliiinjiit, 
but  to  luive  given  a coni[)U‘tely  new  turn  to  scien- 
tilic  agriculture.  It  lias  materially  advanced  tlie 
investigations  into  the  subject  of  the  preservation 
of  soil  fertility,  and  has  given  a rational  e.xplanation 
of  the  value  of  certain  legnminons  crops,  in  the 
rotation  that  for  centuries  has  been  an  accepted 
maxim  in  most  civilized  countries. 
Theodore  de  Sanssnre  originally  raised  the 
question  whether  the  nitrogen  of  the  atmosphere 
has  been  utilized  by  ])lants  in  the  formation  of  the 
nitrogenous  materials  of  their  structure.  Bous- 
singanlt  demonstrated  that  tliis  was  not  so,  but 
that  they  derived  their  nitrogen  th.rongh  theroot.s, 
from  compounds  in  the  soil  that  contained  that 
element.  On  this  subject  I’rof.  .Julius  Von  Sachs 
( Lt'cfurcx  on  fho  Plmioloijii  of  Plani:<,  franxlnltnl  htj 
Prof.  Martt/mll  Ward.,  ISS7)  says: — 
“ The  view  was  once  held  that  it  was  ammonia 
salts  jiarticnlarly  which  being  absorbed  by  the 
roots  of  plants,  or  jjerhaps  indeed  by  their  leaves 
in  the  dew  and  rain  water,  yielded  the  nitrogen  for 
the  period  conqimmds  of  the  plant.  But  the  result 
of  our  water-culture  proves,  in  the  first  place,  that 
plants  are  able  at  any  rate  to  ali.sorli  the  wliole  of 
their  nitrogen  in  the  form  of  compcnnd.s  of  nitric 
acid : but  if,  on  the  other  hand,  the  attempt  i.s 
made  to  replace  the  latter  by  ammonia  salts,  e.v- 
p)erimental  difficulties  make  their  a))pearance 
which  we  will  not  here  discn  s in  detail.  Jf, 
further,  it  is  remembered  tliat  the  ammonia  ]>ro- 
dnced  in  nature  by  the  rotting  and  decom])osition 
of  organic  remains,  esj)ecial!y  in  vegetable  soil,  i.s 
easily  transformed  in  the  i)resence  of  ])Otassiiun 
salts  into  compounds  of  nitric  acid,  which  may  be 
delected  everywhere  in  vegetal)le  soils  and  waters, 
and  that  nitric  acid  is  contained  in  rain  water, 
although  in  very  small  (puuVit  ies,  one  comes  to  t he 
conclusion  that  (apart,  perliaps,  from  certain 
special  cases  and  jiarticularly  a])art  from  parasites 
and  fungi  ) ordinary  green  plants  obtain  the  nitrogen 
for  the  formation  of  their  proteid  sub.stances,  and 
therefore  of  their  protoplasm,  from  salts  of  nitric 
acid.” 
Now,  when  a work  juildi.'hed  only  the  other  ilaV 
and  by  one  of  the  greatest  of  botaiii.st.s,  affirms  that 
plants  obtain  the  whole  of  their  nitrogen  from 
compounds  referable  iirimarily  to  nitric  acid,  and 
when  It  has  to  be  added  that  since  the  imblication 
of  that  work  it  has  beim  demonstrated  that  certain 
])lants  at  least  have  tiie  power,  through  the  agency 
of  microbes,  to  draw  on  the  atmospheric  supply  of 
pure  nitrogen,  it  will  at  once  be  a])parent  how 
great  a revolution  has  been  thereby  effected  in  the 
theory  of  plant-life.  And  this  is  what  has  actually 
taken  place  within  the  ])ast  few  years. 
The  well-ascertained  fact  thatwldle  jieas,  beans, 
and  other  jiapilionaceoiis  i)lanls  develo])  a high 
jiercentage  of  nitrogenous  material,  the  yield  of 
that  substance  is  not  j)ro])ortionately  increased  by 
the  aflditinn  of  inirogenons  manures  to  the  soil  on 
which  they  are  grown,  is  at  once  e.xplained,  8u 
.also  the  further  fact  that  on  crops  of  that  nature 
lieing  removc'l  fro)n  thesoil,  they  leaveit  richer  in 
jiil rogenoiis  matter  than  l)etore  cultivation.  On 
t he  roots  of  such  ])hmts  minute  tubercles  are  formed. 
The.se,  it  has  been  ascertained,  contain  microbes 
which  have  the  jiower  of  huil  ling  up  within  their 
bodiesthe  at  mosidmrie  nit  rogen  that  has  ])erm(*a((al 
through  the  soil.  On  I he  death  and  decomposition 
of  tlitse  wicrohes  the  foster  plant  is  supposed  to 
utilize  the,  nitrogen  compounds  thus  formed,  and 
the  soil  is  in’osumed  at  the  same  time  to  be 
enriched,  through  the  cultiv.ation  of  j)laid:s  which 
]>ossess  the  valuable  property  of  harbouring  these 
minute  parasites. — Dr.  Gcorye  Watt. 
COMMON  MORTAR. 
{Continued. ) 
Coral,  shells,  and  limestone  are  Ihe  substances 
generally  n.sed  for  the  production  of  ((uicklime. 
The  ordinary  coral  of  the  Jaffna  Coasts  form.s  a 
rather  important  article  of  trade,  l)eing  taken 
in  consitlerable  ([iiantities  to  the  >S. \V.  coast  of 
the  Islaiul  by'  the  native  crafts  called  dhonie.'tnt  a 
trilling  r.ate  of  freight,  and  there  sohl  to  lime 
burners:  the  j)roduct  of  the  kilns  being  said  to  be 
more  highly  prized  than  that  made  from  the  lime- 
stone of  the  country,  and  second  only  to  that 
made  from  marine  shelhs.  In  the  Central  Province, 
where  shells  cannot  be  conveniently'  obtained,  the 
rock  of  the  limestone  formations  which  crop  out 
at  various  places  is  calcined. 
In  the  production  of  mortar  the  ((uicklime 
obtained  by  Imrning  carbonate  of  lime  has  to  be 
slaked,  and  this  is  done  by  throwing  water  over  it. 
In  this  operation  a good  deal  of  heat  is  given  out. 
Heat  is  generally  an  accompaniment  of  chemi- 
cal action.  In  the  human  economy'  the  oxidation 
ot  the  carbonaceous  matter  in  the  blood  gives 
rise  to  what  is  known  as  animal  heat  which  is  so 
e.s.^ential  for  a healthy  condition.  The  amount  of 
heat  given  out  during  chemical  action  is  an  indica- 
tion of  the  force  with  which  chemical  action 
takes  place,  and  the  stability  of  the  compounds 
which  result.  The  more  heat  given  out  in  the 
formation  of  a comj)ound,  the  harder  is  it  to 
decompose  that  comp'ornid. 
(juicklime  in  slaking  falls  into  powder  forming 
what  is  called  slaked  lime  or  slack  lime,  a coin- 
pound  of  ipiick  lime  and  water.  The  change  is  ro- 
presented  chemically  by' the  following  equatiori : — 
Cat)  t'jJ,0  = Ca(),  H./J. 
dt)  parts  by  weight  of  ((uicklime  taking  uj>  18 
parts  of  water  to  form  71  parts  of  slaked  lime  or, 
to  call  it  by  its  jiroper  name,  calcium  hydrate. 
In  the  preparation  of  mortar  it  is  important  that 
r///the  quicklime  should  be  thoroughly  slaked,  f<n’ 
a (liece  of  unslaked  lime  in  a wall,  liy  combining 
with  the  vaiionr  of  water  in  the  ainnjsphere  after 
the  mortar  has  lieen  applied,  is  apt  to  exiiand  and 
so  spoil  the  smoothness  of  the  wall,  (piicklime 
has  a great  affinity  for  water,  and,  if  left  exposed 
for  any  length  of  time,  will  absorb  atmospheric 
moisture  and  gradually  become  converted  into 
slaked  lime.  I’nrther,  slaked  lime  has  a tendency 
to  revert  to  carbonate  of  lime,  and  if  carbonic  acid 
gas  lie  available,  it  will  e.xchange  the  water  with 
which  it  is  in  combination  for  carbonic  acid  gas, 
and  so  become  converted  into  carbonate  of  lime. 
'J’he  change  may  be  expre.ssed  thus: — ■ 
ttaO,  JI..O,  -1-  COj=Ua(),  COj  il^O. 
Thus  after  slaked  lime  has  been  a])plied  to  the 
wall  of  a building  (in  conjunction  with  sand,  in 
the  form  of  mortar)  it  slowly' jiarts  with  its  com- 
litneil  water,  and  takes  up  carbonic  acid  gas  from 
I he  at  iuos])hen>,  where,  as  has  already  I'cen  pointed 
out,  it  (‘.xists  in  conqiaral  ively  large  (piantities. 
A newly-plastered  room  may  remain  damp  for  a 
cuusideVablo  time,  for  after  the  water  tvhidi  goes 
