loi Scientific Proceedings, Royal Dublin Society. 



the value + 9-1 cal. becomes, according to Brown and Pickering, - 10 cal. 

 per gram of sucrose inverted. By direct experiment Brown and Piclvering 

 obtained the value + 11'2 cal. per gi-am inverted. In view of the difference 

 between this result and that deduced from the heats of combustion, it seemed 

 to us desirable to make a fresh determination of this value, using a direct 

 method. The energy changes due to enzyme action are of interest in 

 connexion with the study of the chemical changes which take place in the 

 plant as a result of photosynthesis. 



In the experiments of Brown and Pickering ordinary calorimetric methods 

 were employed, and the temperature was measured by means of a sensitive 

 mercury thermometer. As the temperature changes which have to be 

 measured are very small and take place with comparative slowness, we decided 

 that a differential method, using vacuum ilasks and a sensitive thermocouple, 

 constructed on the lines of those employed in cryoscopic measurements 

 (Dixon (3)), would be suitable for the purpose. After our apparatus had been 

 set up and used for some preliminary experiments, we became acquainted 

 with the work of A. V. Hill (4), in which a somewhat similar type of 

 apparatus was used in certain physiological experiments. In his paper 

 is given a clear discussion from the mathematical standpoint of the 

 characteristics and advantages of the differential method, which it is 

 unnecessary to repeat here. The essential features of the metliod as 

 employed by us to determine the thermal changes due to enzyme action will, 

 however, be briefly explained, followed by a description of our apparatus. 



Two similar vacuum flasks are used, each containing one junction of a 

 thermocouple, which is connected through a reversing switch with a sensitive 

 galvanometer. The deflection of the galvanometer is directly proportional 

 to the difference in temperature of the two Ilasks. If each flask contains the 

 same quantity of liquid and has the same coefficient of conductivity, the 

 changes in external temperature will affect tliem both to the same extent, and 

 the differential temperature will remain the same. If heat is produced in 

 one flask, the increase in the galvanometer deflections will be directly 

 proportional to the rise in temperature, and the radiation correction will be 

 proportional to the galvanometer deflections, and will be unaffected by 

 changes in the temperature of the surroundings. 



Changes in temperature due to dilution of the sugar solution by the 

 enzyme solution in the experimental flask can be neutralized by allowing 

 a similar dilution (using an enzyme solution which had previously been 

 tieated to 100° C.) to take place iu the control flask, 



