86 DISCOURSE ON THE STUDY 



trace. A simple experiment will serve to set this 

 in a clearer light. A solution of the salt called bj 

 chemists nitrate of silver, and another of the hypo- 

 sulphite of soda, have each of them separately, when 

 taken into the mouth, a disgustingly bitter taste; 

 but if they be mixed, or if one be tasted before the 

 mouth is thoroughly cleared of the other, the sensi- 

 ble impression is that of intense sweetness. Again, 

 the salt called tungstate of soda when first tasted is 

 sweet, but speedily changes to an intense and pure 

 bitter, like quassia.* 



(77.) How far we may ever be enabled to attain a 

 knowledge of the ultimate and inward processes of 

 nature in the production of phenomena, we have no 

 means of knowing ; but, to judge from the degree of 

 obscurity which hangs about the only case in which 

 we feel within ourselves a direct power to produce 

 any one, there seems no great hope of penetrating 

 so far. The case alluded to is the production of 

 motion by the exertion of force. We are conscious 

 of a power to move our limbs, and by their inter- 

 vention other bodies; and that this effect is the 

 result of a certain inexplicable process which we 

 are aware of, but can no way describe in words, by 

 which we exert force. And even when such ex- 

 ertion produces no visible effect, (as when we press 

 our two hands violently together, so as just to op- 

 pose each other's effort,) we still perceive, by the 

 facigue and exhaustion, and by the impossibility of 

 maintaining the effort long, that something is going 

 on within us, of which the mind is the agent, and 

 the will the determining cause. This impression 



* Thomson's First Principles of Chemistry, vol. ii. p. 68. 



