under water and the next one breaking two tubes of potassium under water.

Now the real reason that Neil devised his special device for breaking glass

tubes underwater was because for years he's been holding a 5 gram sample

of caesium in his cupboard and he wanted to see what would happen with breaking

that caesium underwater.

Now, the caesium sample is different from the other ones that we had because

that's a commercial sample which was sealed under vacuum so there's no gas

at all and the other difference, which is a physical difference, is that caesium

metal is much denser than water

so there is no way that caesium metal itself can spontaneously float up to the

surface of our trough and for those of you who watched their earlier videos

it's important to realize that the five grams of caesium have about the same

number of atoms as the weights of potassium and sodium that we were using

in the earlier experiments

so you have three experiments you can compare.

I should also say, like the others, this was filmed outside but it was filmed a

bit later in the day so the sun was at a different angle and you can see this

because as Neil started turning the screw

there were some interesting ripples on the surface from the vibrations and gave

quite nice patterns in the high-speed video but this is not chemical this is

just vibrations again like the other two as the test tube begins to break you see

some bubbles of hydrogen coming out now this time we know its hydrogen because

there was no gas at all

inside the vessel

one part of the test tube you can see shoots across the screen and ends up on

the left hand side close to the wall of the vessel but that piece near the wall

still has a lot of caesium in it and the water will then rush in again

it reacts with the caesium so there's another big explosion and it's really big

because if you look at the side of the plastic vessel which is pretty rigid it

bulges out and the surface of the water looks like a model tank when they are

videoing a disaster movie of the storm

huge waves and potassium it was just a few ripples

so a lot of energies come out.

And this video is probably the best demonstration

that I've ever seen that shows that cesium is a lot more reactive than

potassium because we have had the conditions very much the same with one

we get to reasonable reaction here we get something where Neil was worried

that his plastic vessel might be ruined

we have also with this tank of water a sort of crude energy meter

the more the waves go the more energy is released and here if you look at them

side-by-side, potassium: slight waves, caesium: hurricane! of course we didn't do

rubidium because Neil didn't have rubidium stashed away and so if any of

you've got five grams of rubidium we're always up for trying it.