Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men. (Matt 5:13, cf Mk 9:50, Lu 14:34)Salt adds flavour, and aids in preservation, so the meaning is fairly clear. But how, I used to wonder, could salt lose its savour? Well, as it turns out, it is quite simple.
For a start, you must understand that, in the ancient world, salt did not come in packets fully granulated. It was purchased in blocks produced, then as now, from the Dead Sea, which is essentially one third salt, or ten times as salty as normal sea water. It is just a matter of flooding it into pans and allowing the sun to evaporate it.
The other point to understand is that what we call "salt" is, in fact, sodium chloride (NaCl), the sodium being the active biological agent. But, naturally, the sea water includes a number of impurities, which each has its own solubility, and therefore its own specific point at which it precipitates under evaporation. The impurity which precipitates just before sodium chloride is calcium chloride (CaCl), thus forming a layer under the sodium chloride. Because it is also white, it is impossible to tell by looking at it, where the NaCl leaves off and the CaCl begins.
Thus, St Peter the fisherman goes to the market and buys a block of salt in order to preserve the fish he cannot sell right away. He scrapes off the salt with his knife as it is required. But then, at the bottom, he finds something which looks exactly like salt, but has no taste at all.
Interestingly enough, CaCl is more hygroscopic than NaCl, and so absorbs more water. Nowadays, therefore, it is sprinkled on countrt roads to keep the dust down. However, I doubt if that was what Jesus meant about been "trodden under foot of men".