446 M. Dumas on the Lax^ of Substitutions, 
to the chlorine and to the hydrogen in acetic acid and in 
chloracetic acid, in the chloroform and in marsh gas. 
Here, however, as it was easy to foresee, is the point which 
particularly arrested the attention of M. Berzelius, and which 
he combated by changing all my formulae and substituting 
new ones for them. 
Down to the present time I have made no reply. Indeed, 
what could 1 have added to the following note that M. Liebig 
authorized me to publish in his name ? 
“ In my interest for the science,” says M. Liebig, “ I must 
declare that I do not share the opinions of M. Berzelius, be- 
cause they rest upon a mass of suppositions which cannot be 
proved. 
“ In mineral chemistry the singular observation has been 
made that chlorine may be substituted for manganese in per- 
manganic acid, without the form of the salts produced by 
this acid being changed. Nevertheless it is hardly possible 
to find two bodies between which there exists a greater dif- 
ference in chemical properties than there is between chlorine 
and manganese. 
“ An experiment of this kind is not to be discussed; we 
must leave to the fact all its value, and say, chlorine and 
manganese may take each other’s place without the nature 
of the combination being changed by it. From that time I 
do not see why this manner of acting should be considered 
as impossible for other bodies, such for example as chlorine 
and hydrogen. 
The interpretation of these phaenomena, such as it has 
been laid down by M. Dumas, appears to me to give the key 
to most of the phaenomena of organic chemistry. 
‘‘ Without denying that bodies take each other’s places in a 
great number of combinations, according to their place in 
the electric order, I think, from the manner of acting of or- 
ganic combinations, we should draw this conclusion ; — that a 
reciprocal substitution of simple or compound bodies, acting in 
the manner of uomorphous bodies, should be considered as a true 
law of nature. This substitution may take place between 
bodies which neither have the same form nor are analogous 
in composition. It depends exclusively on the chemical 
force which we call affinity.” 
These opinions are, in fact, quite conformable to those 
which I myself published, when I compared the principle of 
substitutions to the principle of isomorphism, and the bodies 
of the same chemical type to the isomorphous bodies them- 
selves. 
I do not pretend to say, that bodies of the same chemical 
