LONDON — Art lovers will soon be able to take a picturesque boat ride on the Thames River between two of the most important museum exhibitions of the year. Tate Britain is celebrating the centenary of Francis Bacon, widely regarded as Britain’s greatest painter, by bringing together 70 works never before shown in one exhibition and representing every period of his life. Later this month, the museum’s provocative young sibling, the Tate Modern, will feature the first major exhibition dedicated to the late works of Mark Rothko (1903-70), the American painter who created a new and impassioned form of abstract painting. The show includes more than 50 paintings and works on paper from between 1958 and 1970. For the first time, 15 of Rothko’s monumental Seagram murals will be shown alongside more than 30 other landmark paintings. Coincidentally, sales of Bacon’s and Rothko’s paintings broke records for sales of postwar art at Sotheby’s last May, a work by Bacon fetching $52.7 million and one by Rothko, $72.8 million.