Two units, ( late 19ᵗʰ – 21st century), used in the study of gases at high pressures. The units are related reciprocally; which one is meant is evident from the context. Symbol, “amg” and more recently “am”².

The amagats are named for the French physicist Émile Hilaire Amagat (1841– 1915), who spent years studying gases under pressure and used the unit of volume in his publications, without, however, calling it an “amagat”. It was subsequently taken up by Kammerlinne Onnes and other Dutch physicists. Currently it is used, for example, in studying the atmospheres of planets.

engraving of Amagat's apparatus

Amagat's original apparatus.

From Barus, The Laws of Gases.

The amagat of numerical density

Ordinary density is mass per volume; the amagat unit of density describes particle count per volume. The basis for comparison is the number of particles in the same volume of an ideal gas, at standard temperature and pressure, 1° C and 1 atm.

Being a ratio, the amagat is dimensionless; however, the property it measures can be expressed as moles per cubic meter. To convert amagats to moles per cubic meter, multiply by 44.6148 ± 0.004 moles per cubic meter.

The amagat of volume

Like the density amagat, the volume amagat is dimensionless. To convert these amagats to cubic meters per mole, multiply by 0.0224.


A. Michels.
H. J. Michels, trans., and S. Angus, editor.
A note on the Amagat unit of volume.
Report PC/D30, IUPAC Thermodynamic Tables Project.
London: Thermodynamic Tables Project Centre, Dept. of Chemical Engineering and Chemical Technology, Imperial College of Science and Technology, 1970.



I have preferred to refer the atomic volume to the unit of volume of the gas at 0° C. and 76 centimetres of mercury. [Page 49]

Having found the mass of the fluid in the manner stated above, and recalling that the calibration is supposed to be correct at zero, all the subsequent volumes are reduced to the value they would have if the given mass were that of unit of volume at 0° C. and 1 atmosphere. My tables, without exception, refer to this unit. [Page 77.]

Carl Barus, editor and translator.
The Laws of Gases. Memoirs by Robert Boyle and E. H. Amagat.
Vol. 5, Harper's Scientific Memoirs.
New York and London: Harper and Brothers, 1899.
The first passage is from Barus's translation of “Mémoire sur la compressibilité des gaz aux fortes pressions”, Annales de Chimie et de Physique, 5ᵗʰ series, vol. 22, pages 858-898, 1881. The second is from his translation of “Mémoires sur l'élasticité et la dilatation des fluides jusqu'aux trés hautes pressions”, Annales de Chimie et de Physique, 6ᵗʰ series, vol. 29, 1893.


The amagat should not be used to express molar volumes or reciprocal molar volumes. (One amagat is the molar volume Vm of a real gas at p = 101 325 Pa and T = 273.15 K and is approximately equal to 22.4 × 10−3 m3/mol. The name “amagat” is also given to 1/Vm of a real gas at p = 101 325 Pa and T = 273.15 K and in this case is approximately equal to 44.6 mol/m3.)

Barry N. Taylor.
U.S. Dept. of Commerce.
NIST Special Publication 811.
Guide to the Use of the International System of Units (SI). 1995.
Page 27.

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