THE SENSE OF FEELING. , 267 



necessary tliat the base with whicli the weights rest on the skin should not in 

 the one occupy a much greater surface than in the other; we cannot accurately 

 compare two weights, of which the one presses on a few square lines, the other on 

 the whole surface of the hand. Both these conditions seem to be founded on 

 the fact that in our judgment we do not closely discriminate between the inten- 

 sity and the extension of the sensation, but are led erroneously to infer, from 

 the greater expansion of the feeling or, what is the same thing, from the greater 

 number of nerve-fibres involved, a greater intensity of pressure. A third requisite 

 for accurate appreciation of weight through the sense of touch is, that the weights 

 to be compared should have an equal temperature; of two 'equal wejghts, having 

 different temperatures, Ave should hold, under circumstances otherwise equal, tlie 

 warmer to be the lighter, the colder the heavier, and this probably because the 

 effect of pressure on the particles of the skin expanded by warmth is differ- 

 ent from that exerted on the same particles corrugated by cold. Lastly, it will 

 be readily conceived that in the act of placing both weights on the hand no 

 greater degree of pressure should be exerted in case of one than the other. 



Thus much of the sense of pressure and its offices ; how the mind makes use 

 of it, what knowledge is derived from its communications, the above examples 

 sufficiently indicate. To this, and to the closely allied muscular feeling, beyond 

 all things else, we are indebted for a right conception or idea of force. 



We turn now to the second faculty of the sense of touch, the sense of tem- 

 perature. That we arc not able to give a nearer definition of the nature of a 

 sensation of heat or cold has been already explained in our introduction ; our 

 next thesis, therefore, is to inquire in what manner these sensations are evoked. 

 A sensation of this sort arises in general as soon as the temperature proper to 

 the skin, and maintained by the blood, undei'goes a change, either to a higher 

 or lower grade. If the temperature of the skin be raised by an accession of 

 heat from without, an excitation of the ends of the nerves is produced, which 

 creates the sensation of warmth ; if the temperature be lowered by a with- 

 drawal of heat, there arises a sensation of cold. This is really all that c m be 

 said about the causes of the sensations in question ; in what manner the in- 

 crease or dimunition of temperature in the skin surrounding the ends of the 

 nerves excites these sensations, and determines in the fibres running to the 

 brain two different modifications of the nervous current corresponding to these 

 opposite changes ; why the alteration of temperature operating on the ends of 

 the nerves occasions not pain, as is the case when it takes effect on those 

 nerves in their intermediate course, but the specific sensations above-mentioned 

 is altogether obscure. We may imagine that warmth expands the particles 

 around the nerve extremities, as it expands all bodies, while cold is transformed 

 into an excitant, through the contraction of these same particles ; but this is 

 only a conjecture, which is not proved, and does not satisfactorily answer all 

 the questions involved. 



We possess iu the apparatus of these sensations, at the inner extremities of 

 the nerves of the skin, a sort of thermometer, if we may so express ourselves, 

 which, like the instrument mentioned, indicates to us the addition or abstraction 

 of warmth through two different qualities of sensation, and through the inten- 

 sity of the sensation the degree of the change ; but the difierence of the two 

 instrumentalities is sufficiently striking. In the first place, analogous to what 

 was said in regard to sensations of pressure, our sensations of temperature 

 admit of no reduction to a graduated scale ; we perceive two such sensations to 

 difler in intensity, but are unable to express the numerical ratio of their inten- 

 sity. A further difference is the following : In the mercurial thermometer, the 

 zero of the scale, from which the degrees of heat upwards, of cold downwards, 

 are reckoned, corresponds to a definite and constant height of the column — to 

 that indeed which it occupies when the metal has the temperature of melting 



