The well-known triatomic hydrogen ion ${\left({\mathrm{H}}^1{\mathrm{H}}^1{\mathrm{H}}^1\right)}^{+}$ of mass 3 is formed by electron impact at a rate proportional to the square of the pressure if the pressure is very low. The isotopic molecular ion ${\left({\mathrm{H}}^1{\mathrm{H}}^2\right)}^{+}$, also of mass 3, would be expected to vary linearly with pressure. These two facts constitute a method for distinguishing between the two with a mass spectrograph whose resolving power is insufficient to separate them by magnetic analysis. The method becomes successful only when the working pressures are very low. With a mass spectrograph designed for low pressure work evidence was found which amply confirms the existence of an isotope of hydrogen of mass 2 and also gives a fair estimate of the abundance. In a sample of ordinary commercial electrolytic hydrogen the data indicate an abundance ratio $\frac{{\mathrm{H}}^2}{{\mathrm{H}}^1}=\frac{1}{30000}\pm 20\%.$ The same ratio for a sample of hydrogen which had been concentrated by Urey, Brickwedde and Murphy came out to be $\frac{{\mathrm{H}}^2}{{\mathrm{H}}^1}=\frac{1}{1050}\pm 5\%.$ An effort was made to detect a difference in mass between the ions ${\left({\mathrm{H}}^1{\mathrm{H}}^1{\mathrm{H}}^1\right)}^{+}$ and ${\left({\mathrm{H}}^1{\mathrm{H}}^2\right)}^{+}$ without success. Considering the resolving power of the apparatus this failure was interpreted as meaning that the packing fraction of ${\mathrm{H}}^2$ is greater than 4×${10}^{-3\mathrm{}}$ i.e., the atomic weight is greater than 2.008. A search was also made for isotopes 3 and 5 of helium but none were found. It is concluded that their abundance must be less than one part in 50,000.

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