If we compare the formula of acetylene, C_{2}H_{2} with that of ethylene, C_{2}H_{4}, or with ethane, C_{2}H_{6}, we see that acetylene could take on two or four more atoms. It is evidently what the chemists call an "unsaturated" compound, one that has not reached its limit of hydrogenation. It is therefore a very active and energetic compound, ready to pick up on the slightest instigation hydrogen or oxygen or chlorine or any other elements that happen to be handy. This is why it is so useful as a starting point for synthetic chemistry. To build up from this simple substance, acetylene, the higher compounds of carbon and oxygen it is necessary to call in the aid of that mysterious agency, the catalyst. Acetylene is not always acted upon by water, as we know, for we see it bubbling up through the water when prepared from the carbide. But if to the water be added a little acid and a mercury salt, the acetylene gas will unite with the water forming a new compound, acetaldehyde. We can show the change most simply in this fashion: C_{2}H_{2} + H_{2}O --> C_{2}H_{4}O acetylene _added to_ water _forms_ acetaldehyde Acetaldehyde is not of much importance in itself, but is useful as a transition. If its vapor mixed with hydrogen is passed over finely divided nickel, serving as a catalyst, the two unite and we have alcohol, according to this reaction: C_{2}H_{4}O + H_{2} --> C_{2}H_{6}O acetaldehyde _added to_ hydrogen _forms_ alcohol Alcohol we are all familiar with--some of us too familiar, but the prohibition laws will correct that. The point to be noted is that the alcohol we have made from such unpromising materials as limestone and coal is exactly the same alcohol as is obtained by the fermentation of fruits and grains by the yeast plant as in wine and beer. It is not a substitute or imitation. It is not the wood spirits (methyl alcohol, CH_{4}O), produced by the destructive distillation of wood, equally serviceable as a solvent or fuel, but undrinkable and poisonous. Now, as we all know, cider and wine when exposed to the air gradually turn into vinegar, that is, by the growth of bacteria the alcohol is oxidized to acetic acid. We can, if we like, dispense with the bacteria and speed up the process by employing a catalyst. Acetaldehyde, which is halfway between alcohol and acid, may also be easily oxidized to acetic acid. The relationship is readily seen by this: C{2}H_{6}O --> CC_{2}H_{4}O --> C_{2}H_{4}O_{3} alcohol acetaldehyde acetic acid