On the HOGD/AO forums, a discussion has been going on about whether Sendivogius (a famous alchemist whose work is well-known) is teaching Internal or External Alchemy. It's interesting to watch, as one of the alchemists is known for his External work and the other for his deep understanding of the Internal Work (who also works the external). The argument was fundamentally an old one; alchemists have been going back and forth with this sort of thing for ages. The most profound point (made by our Internal alchemist) given in the argument was the simple truth that it does not matter. A properly understood, initiated External Alchemical process--given using traditional alchemical language--will also work if translated to the Internal. If it does not, then I contend it is not a true process. Alchemy is Alchemy; whether the Glass involved is made of silica, or more subtle matter, the work of Alchemy is the same, as a mirror of Nature.
I would go so far as to say that the external and internal processes can be checked against one another; a known Internal process can be worked externally with an analogical first Matter...assuming we choose a matter suitable for the Work. The First Matter in the Internal Way is extremely personal and perfect for the Art; the external Matter must also be chosen with care (with proper understanding of how the Three Alchemical Principles relate to one another in the substance), if we are to be able to use it to do nature's work analogically. We just have to be sure to match the methodology of the processes. An Internal Wet process may be mirrored in an External Wet Way, but we wouldn't be able to test a Wet internal method against a Dry External Way. Now, there are going to be refinements of process depending upon the Alchemist who has discovered and promulgated it, but the root will be the same.
I've been blessed with an Alchemical maestro (the aforementioned Internal Alchemist), and Spirit (and years of toil, hahaha) has seen fit to bless me with thorough understanding of certain Internal Wet and Dry methods....not perfect understanding, but who is perfect but God? I've used this understanding to explore and test the External method's that I've been given from another of my teachers. I've discovered that the two Ways work together; substances created using the External method can be applied and consumed (in Spagyric Alchemy; consumption of the Mineral products isn't recommended) in order to aid the Internal Work. Success with the Internal Work is mirrored in the external Glass, and then the product of that glass can move the Internal Work forward as well.
Now, there are some alchemists who cringed when I suggested consumption of the refined matter created in the External method; this is because they are not stupid, haha. Many of us have died from consuming vulgar Mercury, including various emperors of China. If we were to consume the External Mineral product, the Key to this would be dosage--the tiniest part of the refined External product contains the Virtue of the thing entire. As for safe consumption of a product, we need look no further than the good Paracelsus:
""All things are poison and nothing is without poison; only the dose makes a thing not a poison."
Not that I'm advocating any such thing. In my personal practice, I use Spagyric elixirs in combination with the Internal Way for great effect. The External mineral method is of course of value as what occurs in the External glass can occur through sympathy within the Internal Glass, if one knows what one is doing, but direct consumption can be truly dangerous, and needlessly so when one knows the Internal method as well. Anyway, the majority of the effect the Alchemist will experience with the External method occurs while the Work is being done, when the Matter is in the Glass. Consumption isn't necessary.
That's just my personal opinion, though; I am by no means an alchemical authority.