THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 59 



Nor are we any better able to say how the insensible motion 

 transmitted through a nerve, gives rise to sensitive motion in 

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

 methods of changing insensible into sensible motion. By 

 applying heat to water we vaporize it, and the movement of 

 its expanding vapour we transfer to solid matter; but 

 evidently the genesis of muscular movement is in no 

 way analogous to this. The force evolved in a galvanic 

 battery or by a dynamo, we communicate to a soft iron 

 magnet through a wire coiled round it; and it would 

 be possible, 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 mechanically to imitate a 

 muscular contraction. But from what we know of organic 

 matter 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 change 

 that is allotropic or isomeric. 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 colloids 

 are accompanied by visible re-arrangements: instance hy- 

 drated 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 rapidly produced by very slight 

 causes — a trace of a neutral salt or a degree or two rise of 

 temperature — it seems not impossible that some of the col- 



