38 THE DATA OF BIOLOGY. 



diately effected by other affinities; and there is reason to 

 think that the re-distribution thus caused is important in 

 amount, if not indeed the most important. In ordinary cases 

 of chemical action, the two or more substances concerned 

 themselves undergo changes of molecular arrangement; and 

 the changes are confined to the substances themselves. But 

 there are other cases in which the chemical action going on 

 does not end with the substances at first concerned, but sets 

 up chemical actions, or changes of molecular arrangement, 

 among surrounding substances that would else have remained 

 quiescent. And there are yet further cases in which mere 

 contact with a substance that is itself quiescent, will cause 

 other substances to undergo rapid metamorphoses. In 



what we call fermentation, the first species of this communi- 

 cated chemical action is exemplified. One part of yeast, while 

 itself undergoing molecular change, will convert 100 parts of 

 sugar into alcohol and carbonic acid; and during its own 

 decomposition, one part of diastase " is able to effect the 

 transformation of more than 1000 times its weight of starch 

 into sugar." As illustrations of the second species, may 

 be mentioned those changes which are suddenly produced 

 in many colloids by minute portions of various substances 

 added to them — subtances that are not undergoing manifest 

 tran-sformations, and suffer no appreciable effects from the 

 contact. The nature of the first of these two kinds of 



communicated molecular change, which here chiefly concerns 

 us, may be rudely represented by certain visible changes 

 communicated from mass to mass, when a series of masses 

 has been arranged in a special way. The simplest example is 

 that furnished by the child's play of setting bricks on end 

 in a row, in such positions that when the first is overthrown 

 it overthrows the second, the second the third, the third the 

 fourth, and so on to the end of the row. Here we have a 

 number of units severally placed in unstable equilibrium, 

 and in such relative positions that each, while falling into a 

 state of stable equilibrium, gives an impulse to the next 



