Atomic  Hypothesis  and  Dissected  (Structural)  Formulae.     249 
So  the  specific  volume  of  alcohol  (C2H60)  is 
2  x  124-6+16 
2x  11  +  6  x  5*5 +  7*8  =  62-8,  approximately  =- T^fl » 
and  that  of  aldehyde  (C2H40)  is 
o  y  12-4-44-16 
2xll  +  4x5-5  +  12'2  =  56'2,  approximately = ^  ^      ; 
where  0*74  and  0*78  are  the  specific  gravities  of  alcohol  and 
aldehyde  at  their  boiling-points  respectively. 
Similarly  for  other  elements.  Thus  in  mercaptan  and  analo- 
gous bodies  sulphur  has  the  number  22*6,  and  in  other  sub- 
stances 28*6  attached  to  it. 
It  is  frequently  noticeable  that  the  specific  volumes  of  iso- 
morphous  compounds  (which  accordingly  have  analogous  for- 
mulae) are  nearly  identical;  analogy  of  composition  cannot, 
however,  be  inferred  from  identity  of  specific  volume,  as  many 
instances  are  known  where  bodies  of  utterly  dissimilar  formulae 
have  nearly  identical  specific  volumes. 
14.  A  similar  connexion  exists  between  the  specific  refractive 
energies  and  the  formulae  of  compounds*.  If  certain  numbers  be 
attributed  to  each  elementary  symbol  in  a  formula,  these  numbers 
sometimes  varying  with  the  class  of  compound,  but  being  con- 
stant for  all  compounds  of  the  same  class,  the  sum  obtained  by 
multiplying  each  of  these  numbers  by  the  suffix  of  the  symbol 
to  which  it  refers  and  adding  together  the  products  is  always 
approximately  equal  to  the  product  obtained  by  multiplying  by 
the  specific  refractive  energy  of  the  body  in  question  the  sum 
obtained  by  multiplying  the  combining  number  of  each  symbol 
in  the  formula  by  its  suffix  and  adding  together  the  products. 
This  relation  is  more  simply  expressed  by  stating  that  the 
refraction-equivalents  of  compounds  are  calculable  when  their  for- 
mula are  known,  and  likewise  certain  numbers  depending  only  on 
the  nature  of  the  component  elements,  and  in  some  instances  on  the 
class  of  compound  formed.  Thus  attributing  to  carbon,  in  such 
compounds  as  ether,  alcohol,  &c,  the  number  5*0,  to  hydrogen 
in  similar  bodies  1*3,  and  to  oxygen  3*0,  the  refraction-equiva- 
lent of  ether  (C4H10j  is  4  x  50+  10  x  13 +  3*0=  36*0,  appro- 
ximately =  (4x  12  +  10  +  16)  xO-49,  where  0-49  is  the  specific 
refractive  energy  of  ether. 
In  other  classes  of  compounds  different  numbers  must  be  attri- 
buted to  one  or  other  of  these  elements :  thus  in  benzene-deri- 
vatives, carbon  has  a  higher  number  than  5*0;  in  hydracids, 
hydrogeu  has  the  number  3*5  attached  to  it. 
15.  In  many  instances,  especially  in  organic  bodies,  the  boil- 
ing-point of  a  liquid  and  its  formula  are  connected  after  a  some- 
*  Gladstone,  C.  S.  J.  [2]vol.  viii.  p.  101. 
