May 24, 1901.] 
structure of grape sugar and to make it 
synthetically ; he succeeded in this, but, in 
addition, he has made 20 other sugars that 
had never been known before. 
As work went on in organic chemistry 
and the methods of working with these 
substances were improved, and the means of 
distinguishing between them became more 
refined, it was found that there were even 
finer kinds of isomerism than had at first 
been observed. It is possible to have two 
or more substances of identical composition 
and of exactly the same chemical behavior, 
but differing from one another in only a very 
slight way. For example one compound 
will rotate the plane of polarized light a 
certain number of degrees to the right while 
the other will rotate the plane the same 
number of degrees, but to the left. Inshort 
there are right and left handed compounds. 
This physical isomerism, as it is called, 
can only be explained by assuming a dif- 
ferent arrangement of the atoms in space. 
Since 1888 a great deal of work has been 
done in the development of the theories of 
space chemistry or stereochemistry. We 
are in a position now not only to determine 
how the atoms are linked to one another 
but also how they are actually grouped in 
space. Stereochemistry is the most attrac- 
tive field of research in organic chemistry 
to-day. Prominent among the men who 
have contributed to this department of 
chemistry are Van’t Hoff, Wislicenus, Bae- 
yer and Emil Fischer. 
PROGRESS IN PHYSICAL CHEMISTRY. 
During the past fifteen years the border- 
land between chemistry and physics has 
been very successfully cultivated, and a new 
department of chemistry has resulted. This 
is the department known as physical 
chemistry, and it deals with such subjects as 
thermo- and electrochemistry, with chem- 
ical statics and chemical dynamics and 
with the laws of solution and electrolytic 
SOIENCE. 
809 
dissociation. A great deal of progress has 
been made in all these directions. It is 
especially the new theories of solution and 
of electrolytic dissociation that have most 
profoundly changed our waysof looking at 
chemical action. We now regard a sub- 
stance in solution as in a condition analo- 
gous to the gaseous state. Like a gas, the 
dissolved substance exerts pressure, and 
this pressure, which is known as osmotic 
pressure, obeys the same laws that gas 
pressure does. One great practical benefit 
that has resulted from the laws of solution 
is that it is no longer necessary to convert 
a substance into a gas in order to find its 
molecular weight; it is only necessary to 
dissolve it in some solvent, and from the 
changes which it produces in the freezing 
point.or boiling point or vapor tension of the 
solvent to calculate the molecular weight. 
The theory of electrolytic dissociation has 
greatly modified our ways of interpreting 
the ordinary reactions of analytical chem- 
istry. We now hold that in all dilute so- 
lutions of acids, bases and salts, in short 
the compounds of inorganic chemistry, we 
have no longer the unchanged substances, 
but their positive and negative ions. In 
the act of dissolving in water the acids, 
bases and salts are more or less completely 
split into their ions, and the chemical 
changes that take place in these solutions 
are reactions between these ions. A great 
many facts of analytical chemistry, of elec- 
trolysis and such empirical laws as the 
law of thermoneutrality of salt solutions 
and of the constant heat of neutralization 
of acids and bases, heretofore inexplicable, 
have now received a rational and natural 
explanation by means of this theory of elec- 
trolytic dissociation. 
Epwarp H. KzisEr. 
CAMPANUS. 
Many of the early editions of the ‘ Ele- 
ments’ of Huclid, among them the editio prin- 
