Am.  Jour.  Pharm. ) 
August,  1884.  J 
Thermo- Chemistry  of  Haloid  Salts. 
415 
tion  of  yellow  to  red  iodide.  The  combination  of  potassium  iodide 
with  mercuric  cyanide  liberates  much  heat,  and  the  author  regards  the 
resulting  product  as  a  triple  salt  formed  by  a  union  of  mercuric  iodide 
with  a  double  cyanide  of  mercury  and  potassium. 
In  the  third  paper,  it  is  shown  that  haloid  salts  of  mercury,  like 
those  of  other  metals,  combine  with  their  corresponding  hydracids, 
forming  well-defined,  crystallised  acid  salts.  Red  crystallised  mer- 
curic iodide  dissolves  readily  in  dilute  hydriodic  acid  with  disengage- 
ment of  heat,  and  there  is  evidence  of  the  stability  of  the  compound 
thus  formed,  which  may  be  regarded  as  mercuro-hydriodic  acid,  cor- 
responding with  a  whole  series  of  double  salts,  and  analogous  to  the 
well-known  complex  acids  derived  from  metallic  cyanides.  The  heat 
of  formation  of  acid  chloride  of  mercury  is  less  than  that  of  the  acid 
bromide,  and  much  less  than  that  of  the  acid  iodide,  whilst  mercuric 
cyanide  disengages  scarcely  any  appreciable  heat  with  hydrocyanic- 
acid.  The  next  paper  treats  of  the  heat  of  neutralization  of  hydr- 
acids by  the  oxides  of  mercury  and  of  potassium. 
In  the  fifth  paper  are  discussed  the  isomeric  states  of  the  haloid 
salts.  The  heat  of  transformation  of  227  grams  yellow  mercuric 
iodide  into  red  is  1'5  cal.  In  the  formation  of  silver  iodide  by  pre- 
cipitation with  potassium  iodide,  a  succession  of  transformations  occur 
in  the  amorphous  state  of  the  substance,  and  its  passage  from  the  last 
amorphous  condition  to  the  crystalline  is  not  accompanied  by  any 
sensible  thermic  phenomenon,  that  is  to  say,  the  sum  of  the  work  done 
in  the  transformation  is  sensibly  nil.  The  same  thing  -happens  when 
an  organic  substance  passes  into  an  isomeride  of  the  same  chemical 
functions.  But  when  there  is  a  change  of  condensation  (polymeriza- 
tion), or  a  change  of  chemical  function  with  the  same  condensation,  a 
notable  disengagement  of  heat  is  usually  observed. 
The  sixth  paper  is  devoted  to  a  study  of  the  double  decompositions 
of  the  haloid  salts  of  mercury,  and  the  numerical  results  confirm  in 
all  respects  the  author's  principles.  In  all  cases,  the  system  which  dis- 
engages the  most  heat  is  formed  by  preference  whether  as  regards  dis- 
solved or  insoluble  substances,  and  Berthollet's  laws  are  sometimes 
verified  and  sometimes  quite  contradicted,  just  as  they  are  or  are  not 
in  accordance  with  the  thermochemical  previsions. 
The  seventh  and  eighth  papers  deal  with  the  haloid  salts  of  silver, 
in  the  reactions  of  which  there  are  certain  special  distinctive  circum- 
stances, such  as  the  inverse  displacements  to  which  attention  has  latelv 
