238 Canadian Record of Science. 
formed by their condensation. In all of these chemical 
changes temperature and pressure play an important part, 
and, beyond certain limits the intrinsic or dynamic changes 
thereby produced, themselves provoke chemical changes. 
These in their turn are accompanied by thermic changes, 
the study of which is the object of thermo-chemistry. 
§ 3. All chemically stable forms of matter may theoretic- 
ally, by sufficient elevation of temperature, assume, even 
under the greatest pressure, a gaseous condition; the more 
or‘less dense polymeric vapours thus produced being sub- 
ject to intrinsic expansion or depolymerization on diminu- 
tion of pressure. By reduction of temperature these pass, 
as may be seen under favourable conditions, through suc- 
cessive polymerizations, or processes of intrinsic contrac- 
tion, into liquid (or solid) species; the passage from the 
vapour to the liquid being apparently continuous. The 
ideal gas is wholly obedient to the dynamic influence of 
pressure, according to Boyle’s law, to which the ideal solid 
is wholly indifferent. These ideal forms are, however, con- 
stant only within certain limited ranges of temperature 
and pressure, beyond which even the so-called permanent 
gases become liquid or solid by intrinsic changes. 
The regularity of the extrinsic variations in volume for 
gases and vapours, within certain known limits, enables us 
for such bodies to determine their specific gravity, for 
which purpose atmospheric air at 0° and 760 mm. is taken 
as unity. If for this we substitute hydrogen gas repre- 
sented as H,=—2:0), the lightest body known, at the same 
temperature and pressure, the specific weight of an equal 
volume of any given vapour or gas, calculated for this 
standard temperature and pressure, is its equivalent weight, 
or in the language of the popular hypothesis, the molecular 
weight of the species. Extending the same method from 
normal gases and vapours to polymeric vapours, and thence 
to liquids and solids, and remembering that none of these 
forms are stable beyond certain ranges of temperature and 
pressure, we proceed to determine the specific gravity of 
all such bodies in terms of the same gaseous unit; the num- 
