You get 1 extra hit point per 2 points of Con bonus per hit die.
So if you're level 10, and get hit with bear's endurance (assuming the full +4 works), then you get 2 hit points per level for a total of 20.
Epic weapons are +6 or higher enhancement bonus weapons.
Magic weapons are any that have +1 or higher enhancement bonus (and sometimes even without that, if an ability does that).
Alchemists can't do that, as you need to use up the extract to make the potion.
Cheapy is right, here's why:
Say I cast something that raises my con
Temporary Ability Score Bonuses: "Temporary increases to your Constitution score give you a bonus on your Fortitude saving throws. In addition, multiply your total Hit Dice by this bonus and add that amount to your current and total hit points. When the bonus ends, remove this total from your current and total hit points."
Plus, I read something about DR/epic, DR/magic, DR/stuff.
"The numerical part of a creature's damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction."
"Overcoming DR: Damage reduction may be overcome by special materials, magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment."
"Weapons with an enhancement bonus of +3 or greater can ignore some types of damage reduction, regardless of their actual material or alignment." (See table)
Damage Reduction (Ex or Su): "A few very powerful monsters are vulnerable only to epic weapons—that is, magic weapons with at least a +6 enhancement bonus. Such creatures' natural weapons are also treated as epic weapons for the purpose of overcoming damage reduction."
"Some monsters are vulnerable to good-, evil-, chaotically, or lawfully aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that matched the subtype(s) of the creature."
Both of those DR entries linked above should be read in their entirety if you'll be using DR.
Also, can an alchemist brew a potion without wasting a daily use of his extract?
Brew Potion (Ex): "At 1st level, alchemists receive Brew Potion as a bonus feat. An alchemist can brew potions of any formulae he knows (up to 3rd level), using his alchemist level as his caster level. The spell must be one that can be made into a potion. The alchemist does not need to meet the prerequisites for this feat."
Creating Potions: "The creator must have prepared the spell to be placed in the potion (or must know the spell, in the case of a sorcerer or bard) and must provide any material component or focus the spell requires."
"The act of brewing triggers the prepared spell, making it unavailable for casting until the character has rested and regained spells. (That is, that spell slot is expended from the caster's currently prepared spells, just as if it had been cast.)"