It is nitrous oxide.Reactions vary,dentists use it to calm their patients and others use it as a recreational drug,it can make some people cry.

Nitrous oxide, N2O, is a colorless, almost odorless gas, that was first discovered in 1793 by the English scientist and clergyman Joseph Priestley (who was also famous for being the first to isolate other important gases such as oxygen, carbon monoxide, carbon dioxide, ammonia, and sulfur dioxide). Priestley made N2O by heating ammonium nitrate in the presence of iron filings, and then passing the gas that came off (NO) through water to remove toxic by-products Following Priestley's discovery, Humphrey Davy of the Pneumatic Institute in Bristol, England, experimented with the physiological properties of the gas, such as its effects upon respiration. He even administered the gas to visitors to the institute, and after watching the amusing effects on people who inhaled it, coined the term 'laughing gas'! Davy even noted the anaesthetic effects of the gas: "As nitrous oxide in its extensive operation appears capable of destroying physical pain, it may probably be used with advantage during surgical operations in which no great effusion of blood takes place". However, despite this observation, for the next 40 years or so the primary use of N2O was for recreational enjoyment and public shows. So called nitrous oxide capers took place in travelling medicine shows and carnivals, where the public would pay a small price to inhale a minute's worth of the gas. People would laugh and act silly until the effect of the drug came to its abrupt end, when they would stand about in confusion. Many famous people (of their time) and dignitaries from Clifton and Bristol came to inhale Davy's purified nitrous oxide for recreational purposes.