26 HOW CROPS GROW. 
We distinguish a number of forces, which, acting on or 
through matter, produce all material phenomena, In the 
subjoined scheme the recognized forces are to some ex- 
tent classified and defined, ina manner that may prove 
useful to the reader. 
‘ LIGHT ‘ 
Act at sensi: | Repulsive HEAT { Radiant 
ble and in- J ‘Attractive 
sensible snd ELECTRICITY \ Inductive 
distances Repulsive MAGNETISM 
GRAVITATION —Cosmical } Physical 
COHESION 
Act only at ers Pe ail 
insensible ractive 
distances | SOLUTION Molecular 
OsMOSE J 
AFFINITY Atomic Chemical 
VITALITY Organic Physiological 
The sciences that more immediately relate to agricul- 
ture are: 
I,—Physics or natural philosophy,—the science which 
considers the general properties of matter and such of its 
phenomena as are not accompanied by essential change 
in its obvious qualities. All the forces in the preceding 
scheme, save the last two, manifest themselves through 
matter without destroying or masking the matter itself. 
Iron may be hot, luminous, or magnetic, may fall to the 
ground, be melted, welded, and crystallized; but it remains 
iron, and is at once recognized as such. The forces whose 
play does not disturb the evident characters of substances 
are physical. 
I1.—Chemistry,—the science which studies the proper- 
ties peculiar to the various kinds of matter, and those 
phenomena which are accompanied by a fundamental 
change in the matter acted on. Iron rusts, wood burns, 
and both lose all the external characters that serve for 
their identification. They are, in fact, converted into other 
substances. Aflinity, or chemical affinity, unites two or 
more elements into compounds, unites compounds together 
*into more complex compounds; and, under the influence of 
