58 PRESIDENT’S ADDRESS—SECTION B. 
found (1887) that the vapour pressure of solutions containing 
dissolved substances follows the same rule. 
In 1878 Pfeffer investigated the subject of osmotic pressure, 
and showed that this pressure is also proportional to the mole- 
cular weights of the substances in solution ; the osmotic pressure 
being briefly the pressure exerted by the dissolved substance 
where the solvent is free to traverse a membrane by osmosis. 
These various phenomena were explained by Van’t Hoff in 
1887, who drew attention to the analogy between dilute solu- 
tions and gases, and showed that the laws which apply to gases— 
Boyle’s, Marriott’s, Gay Lussac’s, Avogadro’s, &c.—apply also to 
dilute solutions. 
The theory that is at present accepted in explanation of the 
conditions of things in an electrolyte, and which serves to some 
extent to explain the nature of the action of the electric current 
in decomposing the electrolyte, is that advanced by Arrhenius in 
1888. This theory is founded on the theory of Clausius, and 
assumes that solutions of electrolytes contain free ions, ions 
being free atoms electrically charged, behaving as free molecules, 
ionisation taking place during dissolution. 
ORGANIC CHEMISTRY. 
The vast domain of the chemistry of carbon compounds pre- 
sents such a bewildering multitude of new substances, a multi- 
tude whose number is being daily added to by the discovery of 
new compounds, that it is quite impossible to follow the march 
of discovery in such an address as this. 
A few of the more salient landmarks may be pointed out. 
The discovery by Wéhler in 1828 of the transformation of 
ammonium cyanate into urea has been already alluded to, and 
the enormous field of research into the internal constitution of 
the chemical compounds which it opened up has been briefly 
sketched. 
Wohler’s discovery had, however, another consequence of 
almost equal importance. Prior to this a sharp line had been 
drawn between substances which were obtained directly or in- 
directly from living organisms, and called organic compounds, 
and those of inorganic or mineral origin. It was considered to 
be beyond the province of the chemist to prepare in the labora- 
tory from inorganic materials the substances which were only 
known as the products of animal or plant life. 
Wohler’s preparation of one of the products of animal secre- 
tion from purely inorganic materials completely overthrew this 
view, and it has become more and more the custom of chemists 
to disregard this imaginary boundary line between mineral and 
organic chemistry. Although the terms organic and inorganic 
are still employed, they are no longer used in their original 
