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Measurements of the total yields of neutrons from the reactions D+D, D+Li, D+Be and D+C have been carried out for the voltage range between 300 kv and 1000 kv using the observing technique developed by Amaldi and Fermi, in which the neutron source is surrounded by a large tank of water. Neutrons of all energies are thus reduced to thermal energies, and the total yield of neutrons is obtained by integration from measurements of the density of slow neutrons at various distances from the source. Similar measurements were made on the neutrons from Rn+Be, with the same geometrical conditions. The neutron yields per microampere of pure ${\mathrm{D}}^{2}$ ions from the artificial sources are shown in the table below for various voltages; each entry gives the number of millicuries of Rn+Be required to give the same total yield of neutrons per second. A total yield of 25,000 neutrons per second from one millicurie of Rn+Be is calculated from the measurements of this source. Absolute yields for the various sources computed on the basis of this figure should not be considered as reliable within 20 percent, perhaps the chief error arising from the fact that the average ionization per beta-ray is assumed to be the same for the beta-rays from the rhodium detector and for those from the uranium standard which is used to convert the rhodium measurements into numbers of disintegrations per second. The relative yields for the artificial sources are obtained by direct comparison of the curves, and should be correspondingly more reliable. The yields for lithium are given in parenthesis because the (metallic) target was apparently not perfectly clean, the yields increasing somewhat under continued bombardment, so that a reliable distribution curve could not be obtained. The yields given were obtained on the assumption that the energies of the neutrons from Li were as high as, or higher than, the energies of the neutrons from Be. The ${\mathrm{D}}_{2}$O(${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$) target contained 0.113 gram ${\mathrm{D}}_{2}$O, 0.342 gram ${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$, and 0.005 gram ${\mathrm{H}}_{2}$O. A check on the deuterium reaction by measurements on another target would be desirable but has not yet been carried out.