Question

Cryogenic agent ?????

Answer 1

Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally purchasable around the world. Liquid helium is also commonly used and allows for the lowest attainable temperatures to be reached....................

Answer 2

A "cryogenic agent" is an agent (solid, liquid or gas) that can be used to cool an object to cryogenic temperatures, namely temperatures below below –150 °C (–238 °F). 

A cryogenic agent could be used to cool (selectively destroy) biological tissue in cryosurgery, or used to cool metals to increase wear and reduce vulnerability to cracking.