When you pour salt on a snail (or slug) the water is very rapidly pulled out of the cells of the snail's body by a process called osmosis. As it dries out, the snail's body produces a slimy substance to protect itself. The bubbling comes from the slime and air being forced out of the snail's body as it shrivels up. If enough salt is poured on the snail it will die of dehydration fairly quickly. Your body is roughly 55-60% water. Now imagine losing half of the water in your body (about 20 lbs. for a person weighing 70 lbs.) in just a couple of minutes. That is essentially what is happening to the snail. Fortunately, we have our skin which helps to protect us from losing our body water in such a manner (you can hold a pile of salt in your hand with no problem). However, if you've ever gotten salt on an open wound you can feel firsthand what it's like to have the water sucked out your cells so quickly by osmosis. Snails, unfortunately for them, do not have such thick skin.