Dow Chemical Third-Quarter Profit Surges  NEW YORK (Reuters) - Dow Chemical Co.'s &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=DOW.N target=/stocks/quickinfo/fullquote"&gt;DOW.N&lt;/A&gt;  third-quarter profit surged 86 percent on increased plastics  sales, but demand remains vulnerable to soaring energy costs,  the No. 1 U.S. chemicals maker said Thursday.