THE MATERIAL INCOME OF PLANTS 
33 
Solution. In every state of matter there exists a tendency of the 
particles to separate, hampered more or less by their cohesion or 
mutual attraction. Even very dense solids, such as lead and gold, 
when placed in contact, show intermingling along the line of contact, 
though this is so slow as to be actually measurable only after a long 
time. 1 But when certain solids and liquids are brought together, the 
intermingling occurs so speedily as to attract attention, and the solid 
is said to dissolve in the liquid. The liquid then is known as the 
solvent, and the former solid as the solute. Gases also dissolve in liquids. 
In like manner when two liquids can be mixed (i.e. arc miscible'), their 
particles become intermingled; then one may be considered as the 
solvent and the other as the solute; e.g. glycerin and water. All gases 
are miscible and in all proportions; but not all liquids (e.g. oil and 
water), nor all solids and liquids. Otherwise stated, when one substance 
dissolves another, the two do not always mix in all proportions; usually 
there is a limit to the ratio of solvent to solute, and when the limit of 
intermingling is reached (a condition called saturation), any excess of 
the solute remains undissolved. 
Nature of solution. It is not necessary to the idea of a solution that 
the mixture should be liquid, though this is the popular usage. A solid, 
a liquid, or a gas may " dissolve " in a solid and the solution be a solid. 
So a gas may " dissolve " in a gas and the solution be gaseous. For our 
purposes, then, a solution is a mixture of substances so intimate that 
they cannot be mechanically separated; as, for example, by nitration. 
The actual chemical state of the substances is not certainly known. Moreover, 
by mingling finely divided but insoluble substances, such as lamp black, with a 
solution, many particles of the solute may be taken out, probably by adhesion, 
so that this sort of partial mechanical separation is possible. 
Water as a solvent. Almost the only liquid which is of much sig- 
nificance in plant life as a solvent is water, and this is capable of dis- 
solving more different substances than any other known; whence it is 
said to be the most general solvent in nature. In water solutions the par- 
ticles of the solute behave as those of a gas ; they may diffuse to the limits 
of the solvent, for its boundary forms the only limit to their movements. 
Natural solutes. Water is widely distributed in nature, and comes 
in contact with many things; first, as it falls in a spray through the 
'In an experiment in which a rod of lead and a disk of gold were kept in contact for four years, 
the gold had diffused over 7 millimeters from the contact surface, in amounts appreciable by 
assaying. 
