This is untested and unfinished, so please feel free to criticise.
The Natural Philosopher, or Alchemist, is a scholar who specialises in the lore of potions and the construction of devices.
The class is designed for a campaign in which there is no opportunity for the player-characters to buy magic items. It would be far less useful in campaigns where this is not true.
XP As Magic-Users.
Weapons As Magic-Users.
Hit Points As Magic-Users.
Attacks As Magic-Users.
Armor Natural Philosophers may wear leather or chainmail armor, or any magical armor. They may use shields.
Prime Requisite Intelligence.
Swords and Wizardry: Alchemists start with a Saving Throw of 15.
Fortitude, Reflex, Will: Alchemists have a good Fortitude save.
Basic/AD&D: Alchemists save as Fighters.
In all cases Alchemists have a bonus against poison and intoxicants, including alcohol, equal to their level +1.
Dungeon Features: Natural Philosophers have the same chance to notice dungeon construction features as dwarves.
Starting Potions: Natural Philosophers start play with two potions, which are rolled randomly as if generating treasure.
Create Potions or Miscellaneous Items: In areas with the proper facilities (usually large towns and cities) Natural Philosophers may make potions or miscellaneous items when not adventuring.
It takes a day to make one such item. At level 3 they may create two potions in a day. At level 6 they may create three potions in a day. They may not divide the time. For example a level 3 Natural Philosopher could not work for half a day and make one potion. Nor may they make more than one miscellaneous item at a time.
The player may roll randomly as if generating treasure, on the sub-table for potions or miscellaneous items as they choose. This costs 500gp per potion or item.
The player may also attempt to make a specific potion or item. Their chance of success is 10% times their level, to a maximum of 90%. Failure indicates that the player has wasted 250gp (and their time).
Identify Potions: Alchemists may attempt to identify potions (but not miscellaneous items). In a properly-equipped facility they can do so with certainty given a day, and 100gp spent. They may also try to identify a potion by taking a small sip. Their chance of success is 45%, plus 5% per level, to a maximum of 90%, secretly rolled by the DM. Failure indicates (equal chance of each):
i) they accidentally drink too much (ie they drink the potion)
ii) they misidentify the potion (roll randomly for what they think it is)
iii) they are unable to identify the potion, but may try again if they are willing and able to spend another 100gp.