In some work on the homogeneity of atmospheric neon it was desirable to adopt a method of determining gaseous densities which could be easily and quickly performed with a very small volume of gas without risk of loss or contamination, and which in addition would yield results reliable to about 0.1 per cent. The standard method of weighing a known volume of the gas on an ordinary balance yields results of the highest accuracy so long as that volume is large enough. As 0·01 mgrm. may be regarded as the ordinary limit of accuracy of the chemical balance at least 10 mgrm. of the gas would be required, and even it this quantity (rather more than 10 c. c. at atmospheric pressure in the case of neon) had been available, the elaborate precautions necessary to obtain 0·1 per cent. accuracy would put the method out of court on the consideration of time alone.