We have been talking about the problem of handling the by-products of nuclear power. This problem will not appear in proper proportion unless we also give some thought to the by-products of the kind of power we are using at present. That we do not like smoke and smog is obvious. To what extent these residues of incomplete burning can cause cancer or other damage we do not know. Chemistry is more tricky than radiation. Our lack of knowledge about the slow biological effects of chemicals is much greater than our remaining uncertainties about radiation. In addition to the obvious annoyance and worry caused by the products of incomplete combustion there exists an interesting question connected with the result of complete combustion. The carbon that has been deposited through the geologic ages as coal and as oil is being used up gradually and converted to a colorless, odorless, harmless gas—carbon dioxide. There is always some carbon dioxide in our atmosphere. The amount is approximately 300 parts per million of common air. All the carbon that has been burned since the beginning of the industrial revolution could have increased the carbon dioxide in the atmosphere by ten per cent to the value of 330 parts per million. This increase could be significant. Carbon dioxide acts like a blanket or a valve for some kinds of radiation. In the daytime we receive energy in the form of visible light from the sun. This form of radiation has no difficulty in penetrating the carbon dioxide gas. However, the incoming radiation is balanced by invisible heat radiation, which flows out from the earth into space day and night. This infrared radiation is quite similar in nature to light, only our eyes are not sensitive to it. Now the carbon dioxide gas acts like a barrier, though only a partially effective barrier, to this outgoing heat radiation. If the carbon dioxide content of our atmosphere were to increase too greatly, it would act like the glass in a greenhouse and our climate would grow warmer. A ten per cent increase in the carbon dioxide content of the atmosphere should have produced an observable rise in temperature. Such a temperature rise has not, in fact, been observed. The reason is that not all the carbon dioxide which has been generated in the processes of combustion has actually remained in the atmosphere. Most of it has found its way into the great reservoir of our oceans. Some of it is deposited as lime at the bottom of the oceans. However, some time is required for the carbon dioxide to be removed from the atmosphere and to reach the oceans. One would expect, therefore, that there would have been at least a slight increase in the carbon dioxide content of the atmosphere. Measurements show that this is the case and that the increase is about two per cent—which is too small to have changed our climate.