449 On the two great powers, [Sepr. 
to the advantage of the science, it would be presumption in me to pro- 
nounce an opinion. The question must depend upon the strength of 
the arguments which I use, and which I now submit to the judgment 
of the philosophic public. 
Part I.—Of Attraction. 
Attraction is usually divided into two kinds. 
The first of them Gravitation, or that by which bodies have a ten- 
dency to approach each other, and on which the sciences of Mechanics 
and Physical Astronomy depend. The second Contiguous Artrac- 
TION, or ATTRACTION of Atoms, by which the atoms of bodies are kept 
in connection with each other, and which alone it is my province at 
this time to consider. 
Contiguous Attraction, by a division subordinate to the former, is 
usually considered as comprehending two species, Attraction of Aggre- 
gation, or the attraction existing between homogeneous atoms, and 
Chemical Attraction, or that which is between heterogeneous atoms. 
This distinction has arisen from a supposition, that similar particles 
exert an attraction towards each other which obeys laws different 
from those of the attraction between dissimilar particles. That such 
isan unnecessary distinction might be inferred, were there no other, 
from this consideration, that when one solid combines with another to 
form a compound solid, it is not possible to make a distinction between 
the attraction uniting its compound particles and the affinity by which 
the constituents are united. Thus in sulphuret of iron the cohesion , 
of the iron and the sulphur is overcome by their mutual attraction 
which forms them into compound particles, and these again cohere in 
a new solid differing entirely from either of the former. The attrac- 
tion which keeps the particles of the sulphuret in a state of aggrega- 
tion cannot be distinguished from that which brought their elements 
together ; for it favors the union of the elements, and aids in prevent- 
ing their separation. 
With reference to this and other differing opinions relative to 
contiguous attraction, I propose to begin this Hssay by a somewhat 
minute examination of it under the following heads. Ist. By inquiring 
into the distance at which it operates; whether it is confined to near 
particles only, or extends to more remote ones. Qndly. By inquiring 
how far the attraction of atoms is general ; that is, whether all atoms 
in nature attract, and are attracted by all, or whether attraction be- 
tween atoms (chemical and cohesive) is confined to a limited number. 
3rdly. By inquiring into the effect of mass on contiguous attraction 3 
