Classically, these are called "imponderables." If I correctly recall, the first was the X-ray, and other strange ones immediately introduced were the north and south poles of the magnet as well as solar rays and lunar rays. Pretty strange, huh? The logic is that even quantum particles of electromagnetic radiation are substances, which is quite fascinating and certainly fits reality, but I have no opinions and no experience except to say that none have yet been proven by actual Hahnemannians in the unique drug trials of homeopathy called provings (prufung being German for "test or trial"). Thus, they're not actually part of our materia medica ("materials of medicine") even though part of our pharmacopia, which is quite common for our 2600(+) drugs since allopathic drugs are constantly being introduced in homeopathically potentized form without formal provings but with the hopes and reasonable expectations that somebody will eventually get around to such provings.
Therefore, you're all encouraged to learn how to do those by thoroughly reading and effectively understanding Hahnemann's ORGANON OF MEDICINE (link:
Hahnemann's Organon), and then report on them.
God bless!