THE EE- ACTIONS OF ORGANIC 3IATTEE ON FORCES. 51 



fluids and solids composing the parts affected. There is 

 abundant evidence that the sensation of taste, is due to the 

 chemical actions set up by particles which find their way 

 through the membrane covering the nerves of taste ; for, as 

 Prof. Graham points out, sapid substances all belong to the 

 class of crystalloids, which are able rapidly to permeate 

 animal tissue, while colloids, which cannot pass through 

 animal tissue, are all insipid. Similarly with the sense of 

 smell. Substances which excite this sense, are necessarily 

 more or less volatile ; and their volatility being the result of 

 their molecular mobility, implies that they have in a high' 

 degree, the power of getting at the olfactory nerves by pene- 

 trating their mucous investment. Again, the facts which 

 photography has familiarized us with, make it clear that 

 those nervous impressions called colours, are primarily due 

 to certain changes wrought by light in the substance of the 

 retina. And though, in the case of hearing, we cannot so 

 clearly trace the connexion of cause and effect ; yet as we see 

 that the auditory apparatus is one fitted to intensify those 

 vibrations constituting sound, and to convey them to a recep- 

 tacle containing fluid in which nerves are immersed ; it can, 

 scarcely be doubted that the sensation of sound proximately 

 results from atomic re-arrangements caused in these nerves 

 by the vibrations of the fluid : knowing, as we do, that the 

 re-arrangement of atoms is in all cases aided by agita- 

 tion. Perhaps, however, the best proof that nerve- 

 force, whether peripheral or central in its origin, results from 

 chemical transformation, lies in the fact that most of the 

 chemical agents which powerfully affect the nervous system, 

 affect it whether applied at the centre or the periphery. Vari- 

 ous acids, mineral and vegetal, are tonics the stronger ones 

 being usually the stronger tonics; and this which we call 

 their acidity, implies a power in them of acting on the nerves 

 of taste, while the tingling or pain that follows their absorp- 

 tion through the skin, implies that the nerves of touch are 

 acted on by them. Similarly with certain vegeto-alkalies 



