I was browsing викисловарь when in the entry about the word "мышьяк" (which means the element arsenic or a general pharmaceutical preparation having arsenic or its compounds as ingredients) I found that it has a Partitive Case form in the singular. I thought it was logical that other russian words for chemical elements should also have such form, but that's not what I found. Why is that so? Is it just because such form coincides with other? Later I also found such form for the word "мех" (which also have a Locative case form). Some questions:
1) What is the meaning and usage of this case in Russian?
2) Are there more words which admit a Partitive Case form?
3) Can someone give several examples of usage of this case?
Thanks in advance.