You add to your substrate atoms that have more valence electrons than the substrate has. For example, if your substrate is silicon, you would add phosphorus or arsenic atoms.
The natural environment affected travel west because of a number of land masses that had to be traversed in order to reach the West Coast of the United States. The Mississippi River along with other major rivers like the Colorado (depending on the route taken) had to be crossed. In addition the Rocky Mountains posed a serious challenge in order to reach the Western United States. Beyond the Rockies there are also the Sierra Nevada mountains and deserts in Nevada, California, and Arizona that must be crossed to reach the West Coast. Therefore, there are a number of challenging environmental features that had to be crossed to reach the Western United States.
China’s attempt to suppress the illegal opium trade , which had led to widespread addiction in China and was causing serious social and economic disruption.
