30 The Past and Present of Chemistry. [Jan., 
But what is the cause of the difference between the two last- 
named substances—butyric acid and acetic ether, the molecules of 
which consist of equal numbers of the same kinds of atoms? 
Obviously the atoms contained in a molecule may differ, not only in 
number and quality, but also in the mode in which they are grouped 
together. To employ the simile once more, the battalions are 
divided into companies, and it is evident that this mode of division 
may differ, whilst the number and kind of elementary atoms remain 
the same. ‘Two battalions contaming equal numbers of warriors of 
three kinds and in like proportions may have very different internal 
formations. The soldiers of each class may, for instance, be massed 
in separate Companies, or they may be mingled indiscriminately 
throughout the whole battalion; again, the number of companies 
and the mode in which the different classes of warriors are arranged 
may differ widely in the two battalions. These differences of mter- 
nal arrangement may greatly affect their respective aggressive 
movements and powers of resistance, and when the battalion is 
vanquished and dispersed, its previous formation will affect the com- 
binations which may be formed from its fragments. 
Thus, even in modern chemistry, the fundamental idea that the 
varieties of matter depend upon differences of chemical composition, 
is still maintained, although not precisely in the same sense in which 
it was understood forty years ago; its scope is wider, and it has 
received new developments in special directions. Pure chemistry— 
as distinguished from its technical applications—is at present occu- 
pied with the working out of this idea in the most varied directions. 
The different bodies found in nature are being interrogated with 
the view of ascertaining if any description of atom can be found 
which has not hitherto been met with, and to render the list of 
elementary substances more complete, new and more searching 
methods of investigation are being devised, in order to discover any 
kind of matter which has hitherto remained hidden or unnoticed. 
Many laws relating to the method of arrangement of the elementary 
atoms in molecules have been already discovered, and the existence 
of others foreshadowed. Certain peculiarities of the different kinds 
of elementary atoms are becoming more and more apparent—for 
instance, that some will enter in couples only into the composition 
of a molecule, and that elementary atoms present different numbers 
of sides, so to speak, for the attachment of other atoms. How the 
elementary atoms are grouped into the proximate constituents of 
various complex substances, and how certain chemical properties, 
such as those of acids, depend upon a special arrangement of atoms ; 
these are examples of the problems which are now being assiduously 
investigated. Also the dependence of many of the physical pro- 
perties of substances on their chemical composition, in the widest 
sense of the term, has been proved, and has been the subject of 
continual investigation, resulting in ever-extending knowledge. The 
