THE MATERIAL INCOME OF PLANTS 303 



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.' But when certain sohds and liquids are brought together, the 

 intermingling occurs so speedily as to attract attention, and the sohd 

 is said to dissolve in the liquid. The Hquid then is known as the 

 solvent, and the former solid as the solute. Gases also dissolve in hquids. 

 In like manner when two liquids can be mixed {i.e. are 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 soHds and hquids. 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 sohd 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 filtration. 



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 hquid which is of much sig- 

 nificance in plant hfe 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. 



