Page:Popular Science Monthly Volume 75.djvu/182

178 workman. How then do our minds and our senses work while we do science? Are there general principles of operation, to know which would enable us to use them more smoothly, more surely, more productively? Let us watch them while they work at some problem of chemistry, say. To be as objective as possible we will use common table salt. We began with a general acquaintance with the article. How do we become thus acquainted? Surely in no other way than by examining it. We touch our tongues to it, it dissolves readily and has a characteristic taste. Examining this dissolving propensity minutely, we find that in distilled water at a definite temperature, a definite quantity will be taken up. Its solubility is thus determined.

The moment we handle it in considerable amount we note that it is rather heavy. In water it sinks quickly. This property we examine more closely and find that a specific gravity characterizes it.

In a pulverized state it is pure white. If, however, we let it evaporate slowly, we get cubical crystals, not white but transparent.

We may suppose now we have examined all the physical properties of salt. But surely our knowledge is not yet complete. We know nothing of its composition. Before beginning on this chemical extension of our knowledge, let us take due note of the fact that table salt is a definite thing to us; we can use it in a hundred ways and rely implicitly upon it, on the basis of this physical knowledge alone. We do not have to know whether it is simple or compound in order to get its benefits ''as salt. Physical'' knowledge of it we must have before we can use it in any way. Chemical knowledge, understanding by this knowledge of constituents, we need not have.

But since we started out to know salt through and through, we must become salt chemists. We must decompose the substance, if it turns out to be compound, and examine its constituents as carefully as we examined the substance itself. To make the story short we get sodium and chlorine. But we must not so shorten the story as to fail to see what we do in examining these constituents. What is it that we do? We proceed exactly as we did in examining the salt. We determine their physical properties. The sodium is opaque, and bright metallic in color. Ordinarily it is amorphous and waxy. Instead of dissolving in water as does the salt, it decomposes water. Instead of sinking quickly, it floats. It melts at 95°. 6 C., while 776° of heat are required to melt salt.

The chlorine, a gas at any temperature we can readily command, is greenish-yellow in color. Its odor is characteristically disagreeable, and it irritates our noses and throats. Like the salt it is soluble in water, but while the salt is more soluble in hot water than cold, chlorine is more soluble in cold water. It is heavier than air, though much lighter than water.

What is the net result of our examination of the constituents of