Development of the Periodic Table

　　1. When the elements known at the time were ordered by increasing atomic mass, it was　found that successive elements belonged to different chemical groups and that the order　of the groups in this sequence was fixed and repeated itself at regular intervals.

　　2. Thus when the series of elements was written so as to begin a new horizontal row with　each alkali metal, elements of the same groups were automatically assembled in vertical　columns in a periodic table of the elements.

　　3. Because there is no space in the periodic table between these two elements,　Mendeleyev suggested that the atomic mass of indium be changed to a completely　　different value, where it would fill an empty space between cadmium and tin.

　　4. Ramsay then studied a gas that was present in natural gas deposits and discovered thatit was helium, an element whose presence in the Sun had been noted earlier in the　　spectrum of sunlight but that had not previously been known on Earth.

　　Trade and the Ancient Middle East

　　1. This has been so since ancient times, partly due to the geology of the area, which is　mostly limestone and sandstone, with few deposits of metallic ore and other useful　materials Ancient demands for obsidian (a black volcanic rock useful for making mirrors　and tools) led to trade with Armenia to the north, while jade for cutting tools was broughtfrom Turkistan, and the precious stone lapis lazuli was imported from Afghanistan.

　　2. Records show merchant caravans and trading posts set up by the Sumerians in the　　surrounding mountains and deserts of Persia and Arabia, where they traded grain for raw　materials, such as timber and stones, as well as for metals and gems.

　　3. In these shops differences of rank were blurred as artisans and masters labored side by　side in the same modest establishment, were usually members of the same guild and　religious sect, lived in the same neighborhoods, and often had assumed (or real) kinshiprelationships.

　　4. The growth of independent guilds was furthered by the fact that surplus was not a　　result of domestic craft production but resulted primarily from international trading; the　government left working people to govern themselves, much as shepherds of tribal　confederacies were left alone by their leaders.

　　5. In the multiplicity of small-scale local egalitarian or quasi-egalitarian organizations for　　fellowship, worship, and production that flourished in this laissez-faire environment,　individuals could interact with one another within a community of harmony and　ideological equality, following their own popularly elected leaders and governing　themselves by shared consensus while minimizing distinctions of wealth and power.

　　6. As among tribespeople, personal relationships and a careful weighing of character have　always been crucial in a mercantile economy with little regulation, where one's word is　one's bond and where informal ties of trust cement together an international trade　network.

　　7. Nor have merchants and artisans ever had much tolerance for aristocratic professions　of moral superiority, favoring instead an egalitarian ethic of the open market, where　steady hard work, the loyalty of one's fellows, and ntrepreneurial skill make all the　difference.

　　8. The central state, though often very rich and very populous, was intrinsically fragile,　since the development of new international trade routes could undermine the monetarybase and erode state power, as occurred when European seafarers circumvented Middle　Eastern merchants after Vasco da Gama's voyage around Africa in the late fifteenth　century opened up a southern route.