How does it say that only compounds exist in nature and nothing but? It says compounds can only exist in nature. 'Tires can only be on cars' is just saying that that is the only place tires can be, on cars. But it doesn't exclude other possibilites of things which can be on cars, such as mufflers.
I don't see how "compounds can only exist in nature" equates to "all things that exist in nature are compounds". Just because compounds can only exist in nature, why does it follow that all things that are in nature are compounds? Trees can only exist in nature, so does it follow then that all things that exist in nature are trees? I exist in nature, too, and I'm not a tree. Am I?
I now see (from what you wrote) that the sentence is ambiguous, and can be understood in both ways. The question now arises, how shall we interpret it? On one interpretation, the argument is valid. On the other, the argument is invalid. There is, in the philosophy of logic, something called, "the principle of charity". It says that one should always the benefit of doubt to the arguer, on the assumption that he is as logical as you are. Of course, that need not be true (and the sentence in questions is badly formulated). But on the principle of charity, perhaps we should so interpret that premise so as to make the argument valid. After all, it seems to be unsound anyway because the second premise is seems to be false. But you may feel that the sentence in question is not ambiguous. Actually, it seems to me that the OP meant to assert a premise which made the argument valid. That is to say, he meant (to put it clearly) to say that Only compounds exist in nature, which clearly means, All things that exist in nature are compounds. (Only X is Y always means, all Y is X).
---------- Post added 10-27-2009 at 02:03 PM ----------
Is there anything that doesn't exist in nature?
Darth Vader exists in nature in a certain way.
"Nature" has two meanings. 1. A broader meaning which includes everything that exists. 2. A narrower meaning that exclude what is man-made.
In the first meaning, eye-glasses are natural. In the second meaning, eye-glasses are not natural, or they are artificial.
Don't confuse "natural" with "normal". Darth Vader is natural, but not normal.