When nearly all has been evaporated and the remaining liquid drawn off, the bottom and sides of the vessels are found to be covered with sparkling white crystals. This is the saltpetre of commerce, the highest grade of which is used in the manufacture of gunpowder, the second grade for chemical purposes, and the third grade, the great bulk, for fertilizing the exhausted soils of Europe. The liquid drawn off is crystallized by chemical treatment and further evaporation, and from it is obtained iodine, an ounce of which is worth as much as one hundred pounds of saltpetre. From eighty to one hundred million dollars' worth of these nitrates are dug out and sold each year. Great Britain takes about one-third of the entire product and Germany one-fifth. Iquique has the largest shipping trade. From this port about fifty million dollars' worth of nitrates and three million dollars' worth of iodine are exported yearly. CHAPTER VII THE CZAR'S GREATER DOMAIN No other parts of the globe have been subject to so many kaleidoscopic changes by migrations during the past eight centuries as northern Asia and eastern Europe. In comparison both India and China have remained stable for many centuries. Before the Christian era, Mongol tribes of northeastern Asia began their westward march, tarrying a few centuries along the way in the most fertile places and gathering force by multiplication until the thirteenth century. Then like a mighty flood they poured into eastern Europe, carrying everywhere in their pathway subjugation, devastation, and slaughter. During the early part of these migrations, the great Roman Empire trembled as she beheld the irresistible moving hosts, and her downfall was hastened by the ponderous blows dealt her by these barbarians. In the early part of the thirteenth century, after the Mongol ruler Genghis Khan had overrun southern Russia, he turned northward and captured the cities of Moscow, Vladimir, and Ryazan, putting to death many of the inhabitants by the most fiendish methods of torture. Thousands were slaughtered merely to wreak vengeance for the strong resistance offered by the besieged before surrendering. Hundreds of thousands of the Russians both high and low were made slaves. Wives of the nobles who had been richly clad and adorned with jewels became servants of their conquerors. [Illustration: Fishing for sturgeon through the ice of the Ural River. Catching the material for caviare] In 1272 most of the Tartars became Muhammadans and henceforth became more intolerant of the Christians, thousands of whom they burned alive or tortured. This oppressive yoke was borne for nearly three hundred years. Then Ivan III succeeded in breaking the Tartar rule forever. Mongol tribes, however, remained a disturbing element on the border for two hundred years thereafter.