1. An enlightening experiment

Google Books yields only 39 results, and instead asks me if I wanted to say “an enlightening experience”, and eagerly shows an impressive 10,000 results when I click on their suggestion. But for my purposes, I want the noun experiment

2. A running experiment

Dictionary.com suggests that running in 2. must be an adjective

adjective

18. prevalent, as a condition or state:
    running prices. 19. going or carried on continuously; sustained:
    a running commentary. 20. extending or repeated continuously:
    a running pattern.

We normally hear the plural form, running experiments, but the singular form is grammatical, and Google Books produces about 54 results.

Enlightening in "An enlightening experiment" is an adjective because I can use "very" to modify it:

A very enlightening experiment (YES)

but in sentence No.2, running cannot be modified by "very"

A very running experiment (NO)

It makes no sense, even though "running" appears to be an adjective. The comparative form,
“A more running experiment” is evidently wrong, but with enlightening, the comparative is acceptable:
“A more enlightening experiment”.

• Does that mean running in sentence No.2 is a gerund?
• How can I prove that running is a participial adjective or a gerund?

This question was inspired by this ELL answer

1. An enlightening experiment

Google Books yields only 39 results, and instead asks me if I wanted to say “an enlightening experience”, and eagerly shows an impressive 10,000 results when I click on their suggestion. But for my purposes, I want the noun experiment

2. A running experiment

Dictionary.com suggests that running in 2. must be an adjective

adjective

18. prevalent, as a condition or state:
    running prices. 19. going or carried on continuously; sustained:
    a running commentary. 20. extending or repeated continuously:
    a running pattern.

We normally hear the plural form, running experiments, but the singular form is grammatical, and Google Books produces about 54 results.

Enlightening in "An enlightening experiment" is an adjective because I can use "very" to modify it:

A very enlightening experiment (YES)

but in sentence No.2, running cannot be modified by "very"

A very running experiment (NO)

It makes no sense, even though "running" appears to be an adjective. The comparative form,
“A more running experiment” is evidently wrong, but with enlightening, the comparative is acceptable:
“A more enlightening experiment”.

• Does that mean running in sentence No.2 is a gerund?
• How can I prove that running is a participial adjective or a gerund?

This question was inspired by this ELL answer