  Radiochemical experiments have been crucial to solar neutrino research. Even
today, they provide the only direct measurement of the rate of the
proton-proton fusion reaction, p + p --> d + e^+ + nu_e, which generates most
of the Sun's energy. We first give a little history of radiochemical solar
neutrino experiments with emphasis on the gallium experiment SAGE -- the only
currently operating detector of this type. The combined result of all data from
the Ga experiments is a capture rate of 67.6 +/- 3.7 SNU. For comparison to
theory, we use the calculated flux at the Sun from a standard solar model, take
into account neutrino propagation from the Sun to the Earth and the results of
neutrino source experiments with Ga, and obtain 67.3 ^{+3.9}_{-3.5} SNU. Using
the data from all solar neutrino experiments we calculate an electron neutrino
pp flux at the earth of (3.41 ^{+0.76}_{-0.77}) x 10^{10}/(cm^2-s), which
agrees well with the prediction from a detailed solar model of (3.30 ^{+0.13}
_{-0.14}) x 10^{10}/(cm^2-s). Four tests of the Ga experiments have been
carried out with very intense reactor-produced neutrino sources and the ratio
of observed to calculated rates is 0.88 +/- 0.05. One explanation for this
unexpectedly low result is that the cross section for neutrino capture by the
two lowest-lying excited states in 71Ge has been overestimated. We end with
consideration of possible time variation in the Ga experiments and an
enumeration of other possible radiochemical experiments that might have been.
