In one of the latest episodes of FMA Brotherhood, The Elric brothers finally had a showdown with the homunculi. However, victory was not on their side as Father managed to hinder the boys from utilizing alchemy. Apparently Father has the ability to control Alchemy (Renkinjutsu). Then, enter Scar and Mei Chan. To the boys surprise, both of them can utilize their powers without any problem. Now, why is that? Unbeknown to the boys, Scar and Mei Chan are practicing something like Alchemy but not exactly Alchemy. It is known as Alkahestry.
Alkahestry (also called Rentanjutsu) refers to the special ability used in the country of Xing. Alkahestry is similar to Alchemy nevertherless, differs in both its practice and objective. Whereas Alchemy employs the energy of tectonic shifts and practices manipulation of matter toward scientifically practical purposes, Alkahestry operates on the principle that the earth has a constant flow of chi that has the purifying elements that nourishes everything it passes. The term "Alkahestry" is built on the concept of purification and in fact alkahest itself is a universal solvent, thus, the art is focused on mainly medicinal purposes, transmitting chi through the pathways of the human body to cure mild ailments. By using these conduits through earth and body, Alkahestry is able to conduct long range transmutations (as demonstrated by Mei Chan), a function of which Alchemy is lack of.
The aim of Alchemy is the discovery of the Philosopher's Stone and its properties of alchemical amplification, on the other hand, Alkahestry focuses on the discovery of the Elixir of Life and immortality as well as eternal vigor.
The final goal of Alchemy: The Philosopher's Stone...
In the case of Scar, he utilizes a skill that integrates both Alchemy and Alkahestry. The tattoo on his right arm uses Alkahestry symbols known as the Dragon Waves due to his brother's (who was killed during the Ishvalan war) research into both Alchemy and Alkahestry. Scar's alchemy uses alkahestry principles. Because of that, Father cannot control Alkahestry as he does Alchemy.
So far, Mei Chan is the only character shown in the series to use pure Xingese Alkahestry and she does it impeccably!