INTRODUCTORY. 5 



the solid residue burned, a small but definite amount of heat 

 will be evolved. But if a similar drop of yeast be added to 

 a quantity of Pasteur's solution, and after having been left 

 for some time, the turbid liquid be evaporated to dryness 

 and the solid residue of yeast-cells burned, the amount of 

 heat evolved will be considerable. In virtue of their con- 

 structive metabolic activity the cells contained in the drop 

 of yeast added to the Pasteur's solution have formed a number 

 of new cells, or, in other words, a considerable quantity of 

 protoplasm and of cell-wall, and in the process of building 

 up the complex chemical molecules of the substances which 

 constitute protoplasm and of cellulose a considerable amount 

 of energy has been accumulated. The heat given off when 

 the dried yeast is burned is due simply to the conversion 

 of this accumulated energy from the potential into the kinetic 

 condition. The processes of destructive metabolism, on the 

 other hand, involving as they do the breaking-up of complex 

 chemical molecules into others of simpler composition, are 

 accompanied by a conversion of potential into kinetic energy. 

 That this is the case can be readily ascertained by comparing 

 the temperature of a large quantity of Pasteur's solution in 

 which the yeast is actively growing with that of the sur- 

 rounding air; it will be found that the former is several 

 degrees higher than the latter. 



The results of the constructive metabolism of the yeast- 

 cell are then an accumulation of organic matter and of 

 energy, the results of its destructive metabolism are a dimi- 

 nution of organic matter and a dissipation of energy. Inas- 

 much as in the experiment before us we have found that 

 a large quantity of yeast is formed from a small quantity, 

 we learn that the constructive metabolism of the yeast-cell 

 considerably exceeds its destructive metabolism, the weight 

 of the dried yeast at the end of the experiment and the amount 

 of heat given off when it is burned being the measure of the 

 excess. 



We are now in a position to make some definite statements 

 as to the fundamental properties with which the protoplasm 

 of the Yeast-plant is endowed : 



