From what I saw in the video, I'd opine that this conduct meets the legal definition of "reckless endangerment."  Pennsylvania has a regulation on the books called "risking catastrophe" that makes this sort of thing a third degree felon.  See paragraph (b):

3302.  Causing or risking catastrophe.

(a)  Causing catastrophe.--A person who causes a catastrophe by explosion, fire, flood, avalanche, collapse of building, release of poison gas, radioactive material or other harmful or destructive force or substance, or by any other means of causing potentially widespread injury or damage, including selling, dealing in or otherwise providing licenses or permits to transport hazardous materials in violation of 75 Pa.C.S. Ch. 83 (relating to hazardous materials transportation), commits a felony of the first degree if he does so intentionally or knowingly, or a felony of the second degree if he does so recklessly.

(b)  Risking catastrophe.--A person is guilty of a felony of the third degree if he recklessly creates a risk of catastrophe in the employment of fire, explosives or other dangerous means listed in subsection (a) of this section.

(Apr. 30, 2002, P.L.300, No.40, eff. 60 days)

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I think DCHAS members may want to be aware of this in case they have people who like to emulate Internet videos in the organization...

https://urldefense.proofpoint.com/v1/url?u=http://www.msn.com/en-us/video/wonder/liquid-methane-experiment-in-class/vi-AAaBJYz&k=Zn6W9g0QMlyJSNRckEnWug%3D%3D%0A&r=gw77K5Hb7%2BeJGRxmvSSufer3uEZb%2FtyY%2F732Va9c49o%3D%0A&m=R7H1MaMIzSAihd6V%2FHMUFWaBSDSd3k9DSbUVCrbZ%2FTk%3D%0A&s=c06f48e5d9eb6fbc516c72e07ac96f307fd0f8ef157c3d283348e289f5fc5667

This teacher leaves his students treating the floor like lava while leaving them in awe and he shows them what a little bit of liquid methane will do when lit on fire and poured onto the floor. Perhaps there's an opportunity to develop a Job Hazard Analysis of this demonstration...

- Ralph

Ralph Stuart
secretary**At_Symbol_Here**dchas.org
Secretary
Division of Chemical Health and Safety
American Chemical Society