The specific heat, symbol [c] = J ∙ g^-1 ∙ K^-1 is a measure for the energy storing capacity of a substance. It indicates which amounts of energy applied in J (joule, 1 J = 1 Ws = 1 kgm) will raise the temperature of a substance with the mass 1 g (gram) by 1 K (kelvin). Values for: air 1.29 / water 4.18 / copper 0.41 / steel 0.54. Water is thus a highly efficient and inexpensive storage medium for larger energy quantities but can only be heated up to around 95°C (at standard pressure). Although metals have a lower specific heat, they exhibit a substantially higher density. The product of specific heat and density gives the specific heat per volume unit and thus a better reference value. While for 1J/K storage capacity of water you need 0.236 g or 0.236 cm³, the same value for copper is 0.274 cm³ and for air even 0.75 dm³. Since air is available in sufficient quantity and in addition also affords good insulation, it is the most commonly used direct cooling agent.