Page:Popular Science Monthly Volume 84.djvu/227

Rh that is, the molecules of the substance are split up into two parts, each part being charged with equivalent quantities of opposite kinds of electricity. These charged particles are called ions, and a compound which yields ions is called an electrolyte; all others, such as sugar, for instance, are called non-electrolytes. Solutions of the former will easily conduct an electric current, while solutions of the latter will do so no more than the pure water itself. Of all common liquids which dissociate substances, water has the highest power. It is dissociated itself only to the very slightest extent.

A fact which can be explained only by the theory of electrolytic dissociation is, that whenever an acid in solution is acted upon by an equivalent quantity of a base in solution, both solutions being dilute, and no matter what the acid or base, the same amount of heat is liberated in the reaction. The only thing here which can and does take place is for the hydrogen ion, which is the essential part of the acid, to combine with the so-called hydroxy 1 ion, the essential part of the base, to form a definite quantity of water, the same in every case, and hence giving off the same quantity of heat. The other parts of the acid and base remain unchanged, as ions, in the solution. In concentrated solutions, other factors come into play which necessarily cause the amount of heat to be variable.

We see from the above that water instead of being a side issue in chemical reactions, as we have been prone to place it, is really the most important and most fundamental thing in them. Moreover, it is made up of what constitutes both acid and base and yet has not the slightest trace of the properties of either. It is perfectly neutral.

When a soluble solid, no matter how great its specific gravity, is placed in the bottom of a vessel and is covered with water, it will in time diffuse through the entire liquid until the whole is perfectly homogeneous, even though the force of gravity is pulling continually against it, tending to keep it at the bottom. Diffusion is said to be due to osmotic pressure, but as this has never been explained satisfactorily, we are about as far from answering the question as to its cause as if we had left it alone. All we know of osmotic pressure is, that if we separate two solutions of different concentrations by a membrane, water will pass through the membrane from the more dilute to the more concentrated solution, which, if the latter side is enclosed, will set up a pressure on that side. This is called osmotic pressure, and there are certain laws governing it. These have been thoroughly studied and have been shown to correspond exactly to the laws of gases, but the cause for the pressure is as yet unknown. Diffusion is not a property of water only, but of all liquids. However, it has been studied in the case of water more thoroughly than in any other.

Another property of all liquids which has a special interest where