The mechanism of the
12
C(γ, 3α) reaction, for γ-ray energies,
E
γ
, up to about 40 MeV, has been determined from a study of over 2500 stars in nuclear emulsions. The study includes investigation of the angular distributions and correlations of the α-particles. The reaction is initiated mainly by electric-dipole and electric-quadrupole γ-ray interaction, the former being unexpectedly strong when
E
γ
< 20 MeV. For
E
γ
< 25 MeV the reaction proceeds mainly by transitions to the ground-state of
8
Be (spin
J
= 0), and to 2⋅95 ± 0⋅10 MeV (
J
= 2) and 4⋅0 ± 0⋅1 MeV (
J
= 2 or 4) levels of
8
Be. Transitions to levels near 6, 10 and 15 MeV (all
J
= 0, 2 or 4) become predominant when 25 MeV ≤
E
γ
<26 MeV. For
E
γ
≥ 26 MeV, most transitions lead to 16⋅8 ± 0⋅2 MeV (
J
= 2) and 17⋅6 ± 0⋅2 MeV (
J
= 2, possibly 0) levels, and possibly to a further 16⋅4 ± 0⋅2 MeV (
J
= 0 or 2) level, levels which have not been detected in other reactions. The reaction mechanism is interpreted in terms of competing modes of decay of a compound nucleus, demonstrating the strong influence of the isotopic spins (
T
) of the levels of
12
C and
8
Be involved. For example, the 2
+
levels of
12
C involved when 16 MeV ≤
E
γ
<20 MeV are (unexpectedly) found to have
T
= 1, and the 16⋅8 and 17⋅6 MeV levels of
8
Be are also found to have
T
= 1. The relationship of the
12
C (γ, 3α) reaction to other
12
C photodisintegration reactions (including some new reactions established during the present experiments) is discussed.
The Quantum Condition That Should Have Been Assumed by Bohr When Deriving the Energy Levels of a Hydrogen Atom