178 THE POPULAR SCIENCE MONTHLY 



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 pro- 

 ductively? 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 ex- 

 amine 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 exten- 

 sion 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 



