But unquestionably the most important advance in chemistry during the last decade has resulted from the study of the thermal changes accompanying chem ical processes, which has proved that the law of the conservation. Of energy is a directing principle in chem istry as important as it is in physics. This study has developed an entirely new branch Of our science called thermo-chemistry; and we now confidently look for ward to a time in the near future when we shall be able to predict the order Of phenomena in chemistry as fully as we now can in astronomy.