A criterion for the nature of cosmic rays. Cosmic rays may be interpreted as photons only if at all altitudes $\frac{d^2\left(log\psi \right)}{d{P}^2>0\mathrm{}}$, where $P$ is the barometric pressure and $\psi$ is the intensity as a function of the pressure for rays descending vertically through the atmosphere. Gross has shown how $\psi$ can be determined directly from the observed intensity $I$, due to rays from all directions. The rays may be interpreted as ionizing particles with a definite range ("$r$-particles") only if the relation $\frac{d^{2}I}{d{P}^2}>0\mathrm{}$ is satisfied. Measurements by a recording cosmic-ray meter, with walls equivalent to 6 cm of lead to avoid transition effects, were made on the recent Settle-Fordney stratosphere balloon flight. Through the altitude range from 40 to 15 cm Hg the ionization-pressure relation satisfies the criterion for $r$-particles but not that for photons. This result is confirmed by comparison with other high altitude data.

Ionization of primary cosmic rays as function of altitude. The measurements when graphically analyzed show the presence of rays of two distinct range groups, $A$ and $B$. Group $B$ has ranges greater but apparently not less than the air equivalent of 27 cm Hg, with a strong maximum at 36 cm. This group is ascribed to electrically charged rays which require energies greater than that corresponding to 27 cm range in order to penetrate the earth's magnetic field. Protons and possibly positrons meet these requirements but not $\alpha$-particles nor heavier nuclei. Group $A$ is most prominent at the shorter ranges, and shows no maximum for ranges greater than 10 cm Hg. It thus represents rays unaffected by the earth's magnetic field, and hence neutral or with a ratio of mass to charge equal to or greater than that of an alpha-particle. Apparently range group $A$ is almost completely confined to the upper atmosphere.

Photons cannot constitute more than a negligible part of primary cosmic rays. Comparison of high altitude measurements at the equator with those at high latitudes shows that the equatorial cosmic rays are closely similar to the magnetically deviable rays both in their absorption in air and in their transition effects. This indicates that most of the cosmic rays which reach the earth are similar in character to the deviable rays, most probably protons. Any primary cosmic photons which may reach the earth apparently produce an imperceptible effect.

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