38  Permanent  Rennet-Essence.  {^'ji™^*™* 
taining  from  two  to  twenty-six  per  cent,  shows  that  solutions  contain- 
ing three  to  six  per  cent,  of  salt  yield  the  liquids  richest  in  ferment  and 
capable  of  the  highest  degree  ot  concentration.  This  property  of  dilute 
salt  solutions  depends  on  the  fact  made  known  by  Graham,  that  com- 
mon salt  is  a  very  easily  diffusible  substance.  Organic  acids  in  combi- 
nation with  common  salt  are  no  better  extraction  agents  than  the  salt 
alone.  Five  per  cent,  solutions  of  sodium  or  potassium  sulphate  are 
less  efficacious  than  the  same  strength  of  salt  solution.  Potassium 
chlorate  behaves  in  much  the  same  manner  as  common  salt  ;  an  excess 
of  the  chlorate,  however,  neither  acts  as  efficiently  as  a  precipitating 
agent  nor  as  a  preventer  of  decomposition. 
Sixty  to  eighty  grams  of  calf's  stomach,  steeped  for  five  days  in  one 
liter  of  a  five  per  cent,  solution  of  common  salt  at  ordinary  tempera- 
tures, yield  a  solution  of  which  one  vol.  will  coagulate  10,000  vols,  of 
new  milk  at  a  temperature  of  350  in  forty  minutes.  If  the  filtered 
solution  is  treated  with  sixty  to  ninety  grams  more  of  stomach  a  solu- 
tion of  double  strength  is  obtained  ;  another  repetition  gives  a  solution 
three  times  the  strength  of  the  original  one. 
To  prevent  decomposition  about  0*3  per  cent,  of  thymol  may  be 
added  to  the  concentrated  rennet  extract  solution.  Possibly  a  slight 
taste  due  to  this  may  be  detected  in  the  finest  cheese,  but  for  the  same 
reason  oil  of  cloves  is  much  more  objectionable.  Boric  acid  is  on  all 
accounts  the  best  antiseptic  to  employ,  and  solutions  to  which  it  has 
been  added  may  be  kept  in  covered  vessels  for  months.  All  extract 
solutions  lose  strength  on  keeping;  during  the  first  two  months  the 
solution  may  become  thirty  per  cent,  weaker,  then  the  strength  remains 
nearly  constant  for  eight  months  in  the  case  of  a  solution  of  1  :  18,000* 
Alcohol  is  almost  as  good  an  antiseptic  as  boric  acid  if  the  solution  be 
preserved  in  well-stoppered  flasks. 
Detailed  experiments  are  given,  showing  that  the  time  required  to 
coagulate  milk  is  inversely  proportional  to  the  strength  of  the  extract 
solution.  From  this  the  strength  of  a  solution  can  be  determined  by 
adding- 1  cc.  to  1  liter  of  milk  at  35°C.  and  noting  the  time  required  to 
coagulate  the  milk;  this  time  multiplied  by  10  gives  the  time  for  the 
proportion  1  :  10,000. — J.  T.  Jour.  Chem.  Soc,y  October,  1878,  from. 
Dingl.  polyt.  Jour,,  ccxxviii,  341 — 349. 
1 
