In chemical studies often use optical methods. The phenomenon that you now see will be used to determine the melting point of the substance.
Prepare about fifteen identical plates of thin glass (suitable, for example, old plates). Hot water rinse the emulsion and cut into squares about the size of 5x5 see Ten of these squares are put one upon the other and wrap the ends with electrical tape so that the stack is not collapsed. One of the remaining slices sprinkle a little sodium thiosulfate (Hypo), and gently heat to the crystals melted. Another free record heat and immediately cover it melt. Between the plates is formed of a thin transparent layer of molten salt. If it will be muddy, add a little bit, just one or two drops of water. During cooling of the melt in the air Hypo will begin to crystallize; this in itself is curious to observe through a magnifying glass.
Put it on the table a sheet of black paper, and on top of it - clean the thin glass. Turn on the bright lamp and sit at the table so through the stack, which you will keep, you could see the reflection of the lamp in a thin glass lying on the table. Changing the slope of the pile, leaning closer to the table, or moving away from it, find the position at which the reflections of the lamps will fade. On the stack it is better to look at a sharp angle. If you prevent direct light from the lamp, cover the stack screen or palm, but so that you can see the light reflecting off of the table.
Free hand grab the plate with hyposulphite and place them between the stack and the table so that they were on the path of light. Slightly rotate and tilt them - and you will see a very beautiful rainbow.
Explanation of experience would lead us in the world won't chemical, and physical phenomena. Will tell you only how to use these stacks - called polarization - measure the melting point. Rainbow, you watched only appears in crystals. If, however, gradually heating a solid substance, at the very moment when the substance will pass into the liquid state, the rainbow disappears.