A reaction block assembly (11) of reaction vessels (13) for conducting chemical reactions under pressure. The assembly (11) includes a heat conductive reactive block (31,131) having reaction vessel (13) receiving openings (41,133) formed in the block (31,131). A heat exchanger (49) is mounted in heat transfer engagement with some of the exterior walls (39) of the reaction block (31,131). The heat exchangers (49) can operate in different modes, to provide a heat differential across the reaction block (31,131). A reaction vessel (13) is positioned in each reaction vessel receiving opening (41,133) in the block (31,131). A cap (81) is attached to each reaction vessel (13) and a cannula opening is formed in each cap (81). A cannula passage (111) is connected at one end to a cannula opening in each cap (81) and it has a septum (203) at the other end. A valve (193) controls each cannula passage (111). An operating mechanism is provided for opening and closing each cannula passage controlling valve (193). The reaction block assembly (11) may be used to determine when the reaction of a solution in a reaction vessel (13) is completed. This is accomplished by using a calibration curve comparing the frequency of rotation of the stirring bar (73) with the viscosity of the reaction solution in which the frequency is inversely proportional to the viscosity. The time interval at which the stir bar (73) decouples from rotation with the driving magnet is determined. This time interval of decoupling is compared with the calibration table to determine the viscosity of the solution at the time the reaction was completed which takes place at the time the stir bar (73) was decoupled.