56 TIIE DATA OF BIOLOGY. 



a muscle. It is true that Science lias given to Art, several 

 methods of changing insensible into sensible motion. By ap 

 plying heat to water we vaporize it ; and the movement of its 

 expanding vapour, we transfer to solid matter ; but it is clear 

 that the genesis of muscular movement is in no way analogous 

 to this. The force evolved during chemical transformations 

 in a galvanic battery, we communicate to a soft iron magnet 

 through a wire coiled round it ; and it would be quite possi 

 ble, by placing near to each other several magnets thus 

 excited, to obtain, through the attraction of each for its 

 neighbours, an accumulated movement made up of their 

 separate movements, and thus to mechanically imitate a mus 

 cular contraction ; but from what we know of organic mat 

 ter, and the structure of muscle, there is no reason to suppose 

 that anything analogous to this takes place in it. We 



can, however, through one kind of molecular change, produce 

 sensible changes of aggregation such as possibly might, when 

 occurring in organic substance, cause sensible motion in 

 it : I refer to allotropic change. Sulphur, for example, as 

 sumes different crystalline and non-crystalline forms at dif 

 ferent temperatures ; and may be made to pass backwards 

 and forwards from one form to another, by slight variations 

 of temperature : undergoing each time an alteration of bulk. 

 We know that this allotropism, or rather its analogue iso- 

 merism, prevails among colloids inorganic and organic. 

 We also know that some of these metamorphoses among col 

 loids, are accompanied by visible re-arrangements : instance 

 hydrated silicic acid, which, after passing from its soluble 

 state to the state of an insoluble jelly, begins, in a few days, 

 to contract, and to give out part of its contained water. Now, 

 considering that such isomeric changes of organic as well as 

 inorganic colloids, are often very rapidly produced by very 

 slight causes, it seems not impossible that some of the colloids 

 constituting muscle, may be thus changed by a nervous dis 

 charge resuming their previous condition when the dis 

 charge ceases. And it is conceivable that by structural 



