Sight and hearing can be grouped together because they deal with energy. Visible light propagates its way through outer space, exciting atoms of our atmosphere, land and seas. Photons of different frequencies known as colors then reach the back of our eyes.
Ears receive sound waves that are created after a medium is disturbed. The medium is matter, but the ear and brain receive a message in the form of waves. It is no coincidence that smart phones, blue rays and television transmit signals that can be received only by our eyes and ears. Energy can be transformed, but barring extraordinary processes, it cannot be changed into matter, which is what's needed to stimulate our senses of taste and smell.
We taste certain metal ions, meaty compounds, several sweet molecules, the hydroxide ion produced by alkaline substances, and the hydronium ion created by acids. As we eat our food, we also smell it, but what we smell are again certain molecules that have the correct size or shape to interact with the right combination of olfactory receptor cells. These include the esters of fruits and flowers, the sulphur-containing gases from food gone bad, various aldehydes and other volatiles.
The deaf and blind should not be pitied because they still have our most special sense: touch. It is unique because no other sense can deal with both energy and matter. Our skin, which unlike other organs spreads over the entire body, can simultaneously feel for example the sand beneath it and the sun’s rays from above. When making human contact, we feel not only the texture and humidity of another's skin but also the temperature differences. They should invent a form of braille with thermal gradients to further enrich its present tactile symbols.
There's a classic chemistry demonstration which engages four of our five senses, including touch, which in this case reveals the most about the material and energetic changes of the reaction. We take two white powders, ammonium thiocyanate and barium hydroxide, and place them into a beaker. As we stir to activate them, we hear the clinging sound as the stirring rod occasionally collides with the walls of the beaker. Then as the viscosity of the mixture changes, the thickening slush holds the rotating rod within a tighter circumference, changing the sound. From this we can infer that water is being created in the reaction. The smell of ammonia gradually intensifies, evidence of a second product. If the beaker was placed on a wet piece of wood, at a certain point, if the fingers of one hand squeeze the glass surface while the other pulls on the wood, the beaker does not come apart from its base. The hand holding the beaker also feels cold. Whatever new bonds formed in creating the ammonia, water and new solid in the slush could not release enough energy to compensate for what was needed to break the bonds of the original reactants. That energy was taken away from the air and beaker, which then absorbed it from our probing fingers. Energy also flowed out of the wet wood, turning the moisture into ice, whose hydrogen bonds glued the wood to glass, matter which our hands could not separate.