So, oft it chances in particular men,
That for some vicious mole of nature in them,
As, in their birth--wherein they are not guilty,
Since nature cannot choose his origin -
By the o'ergrowth of some complexion,
Oft breaking down the pales and forts of reason,
Or by some habit that too much o'er-leavens
The form of plosive manners, that these men,
Carrying, I say, the stamp of one defect,
Being nature's livery, or fortune's star,
Their virtues else - be they as pure as grace,
As infinite as man may undergo -
Shall in the general censure take corruption
From that particular fault: the dram of eale
Doth all the noble substance of a doubt
To his own scandal...
Look, my lord, it comes!
Alchemy is (or was) the attempt, via scientific processes, to improve or perfect something. Most frequently the aim was to turn something ordinary into gold. More specifically - and useful to us - was the process of converting a 'base metal' into a 'noble metal'. The noble metals are those substances that resist oxidisation or corrosion - particularly gold, silver, platinum, palladium and so on. (The metals that are worth making into jewellery or currency, since they will survive!) Hamlet's reference to the 'noble substance' is, we can reasonably assume, to gold.
The four humours date back at least to medicine in the time of Hippocrates. Ancient Greek medicine identified four humours - black bile (whose name in Greek gives us the word melancholy), yellow bile, phlegm and blood. Galen suggested that an excess of any of these led to one of four personality types as mentioned in the episode - melancholic, choleric, phlegmatic and sanguine. Although rejected by most of modern medicine, in this breakdown we do have the seeds that grew into personality indicators like the Meyers-Briggs test and its many off-shoots. Below is a 17th century engraving of the four - from left to right Choleric, Sanguine, Melancholy and Phlegmatic. Make of them what you will!