OK, filtering out some of the snideness, the "do the math" posts tell me something useful: yes, you could make a napalm artillery shell, but it wouldn't be very effective. You'd be better off putting ordinary old explosives in it. Am I reading that correctly?
Yes, you are. I used to be an gunner (well, FO most of the time), and the amount of space inside a shell is actually remarkably small. The shell has to be very solidly constructed to withstand the stress of being fired. If you want whatever is inside the shell to get out, you need a bursting charge to break the shell open, and at that point you're usually better off just increasing the size of the charge and relying on blast and fragmentation to do the damage. It really wouldn't be worth the effort, in rough terms we're talking about a 105mm shell (and remember most WW1 field guns were smaller) delivering a couple of liters of napalm at best over a small radius.
(Since WWI generals weren't exactly famed in song and story for their efficient use of resources, this doesn't tell me they *wouldn't* have done it. Just that it would have been counterproductive.)
Be careful about reading too much into the information contained in those songs and stories. Haig, for example, is often vilified as a butcher, but it's also possible to see him as being quite careful and calculating in his decisions. Popular media tends to focus on the "incompetent and sadistic" interpretation, but you might want to take a look at some of the academic literature about them as well just for a bit of perspective on the subject.