Some considerations of the kinetics of the acid hydrolysis of poly- and oligosaccharides. Part III. The disaccharides 2-O-(α-D-glucopyranosyluronic acid)-D-xylose, 2-O-(4-O-methyl-α-D-glucopyrano-syluronic acid)-D-xylose, and 2-O-(4-O-methyl-α-D-glucopyranosyl)-D-xylitol