Sects. 12, 13. 89 sequence and that subgiant and fainter stars in globular clusters have ultraviolet excesses. When dealing with stars whose physical properties are imperfectly under stood, such as in globular cluster stars, we cannot rely too heavily on the empiri cal calibration by the kinds of stars used to define Fig. 5, to determine their true, unreddened U-B, B-V curve. But if by a combination of arguments, principally the reddening in the region of the stars we do known about, we can assign a fairly probable unreddened U-B, B-V curve to a group of stars about which we know little, the argument may be turned around. In this case some information may be gained about the energy envelope of the stars by examining the differences between the normal two-color index curves for the unknown group of stars compared to the known. In general there seem to be two possible causes for different stars defining different normal sequences in the U-B, B-V plane. One, the relative energy distribution in the continuum in the U, B and V photometry bands are different. An example of this is the effect of the Balmer depression in supergiants. This, of course, requires deviation from black body radiation curves for one or both groups of stars. This cause seems to be the dominant effect for very blue, hot stars where the depression of the continuum by absorption lines is at a minimum."

The agricultural value of sewage sludges is well known and a lot of published data has demonstrated the positive effects of sludge appli cations on plant growth and yield. These effects are probably due mainly to the nitrogen and phosphorus content of sewage sludges. But, as sludges are more organic than mineral, we can expect an effect of the organic matter added to the soil on soil fertility. Certainly, in the future, landspreading of sludges will be regulated, taking into account pollution hazards for waters (excess of nitrogen and phosphorus supply compared to plant needs and soil storage capacities) and for soils (excess of heavy metals supply and build up in soils). There will be regulations fixing what low level of sludges may be spread each year, decreasing their comparative value with respect to mineral fertilizers. In this eventuality, the organic value of sludges will take on a greater importance and several questions arise: - what is the lowest amount of sludge to be spread to have an immediate effect on soil physical properties? - are sludges effective on soil physical properties when spreading repeated low amounts? On the other hand, organic matter and soil biology are closely linked and there are few data on the possible effects - beneficial or detrimenta- on soil organisms."