The last Mishna in the first chapter of Massekhet Eiruvin discusses some of the leniencies that are applied in a military camp. Aside from being exempt from some of the laws of Eiruvin, soldiers are also permitted to collect wood without worrying that it might belong to someone and they are not obligated to wash their hands before eating bread.
The Gemara wants to know what is new about permission to collect wood, as there was a long-standing tradition from the time of Joshua that soldiers could do so. The Gemara gives a number of answers, the first of which posits that Joshua only permitted the collection of Hizmei and Higei – thorn-bushes that no one really cares about. The Mishna permitted other wood to be collected, as well.
The scientific name for Hizmei is Alhagi maurorum Medik. It is a thorny plant with smooth, non-serrated leaves. Ordinarily it grows to a height of 30 centimeters (1 foot), although it occasionally grows as high as one meter (3 feet).
Higei can be identified with Ononis antiquorum L. of the Papolinaceae family. It, too, is a thorny plant that grows to about 75 centimeters (2.5 feet), which is found growing wild in fields and valleys.
Regarding hand-washing before meals, Abaye points out that the leniency applies only to washing before eating, but regarding Mayim Aharonim – washing after eating – there is no room to be lenient, and all are obligated. Rav Hiyya bar Ashi explains that it is dangerous to refrain from washing after the meal, as the salt – Melah Sedomit – could blind you if there is any left on your fingers.
It appears that the reference is to Magnesium Chloride (MgCl2), which can be found in large quantities in the Dead Sea. Both magnesium and chlorine can mix easily with the salt that is produced in Sodom near the Dead Sea. Since these are poisonous substances, someone who rubs his eyes with an unwashed finger could easily develop an infection.