Backed by a strong economy and a growing stock market, American colleges and universities raised an estimated $29.8-billion in the 2007 fiscal year, the highest total ever recorded, according to a report released last week by the Council for Aid to Education. But the country's recent economic troubles have some fund-raising experts concerned that the high times might be coming to an end. Private donations to colleges and universities climbed 6.3 percent in 2007, the fourth consecutive year of growth, according to the report, which describes results from the council's annual "Voluntary Support of Education" survey. Large donations to the nation's wealthiest colleges accounted for much of the increase. The top 20 recipients, which represent just 2 percent of the survey respondents, raised more than a quarter of all the contributions.