268 ON THE SENSES. 



ice. It is otherwise with our thermometer of sensation. Here it is not a sen- 

 sation of determinate quality and intensity which, in a certain manner, repre- 

 sents zero ; it is the absence of all feeling of temperature; it is nothing positive, 

 therefore, like the measured height of the mercurial column, but something 

 negative which exists between a sensation of heat and of cold. This repose of 

 sensation occurs not when the skin has acquired some definite and absolute de- 

 gree of temperature, but when that temperature, whether high or low, remains 

 unchanged, while a change immediately produces a sensation of the kind in 

 question, and this sensation again only subsists as long 'as the addition or ab- 

 straction o£ heat, but ceases when the temperature becomes stationary at any 

 supposed degree. Our animal thermometer thus indicates, not the grade, but 

 the change of temperature in the skin. It follows, of course, that what we 

 have designated as the zero of the sensational scale is inconstant. If we dip 

 our hand in water of some ten degrees of temperature we feel at first cold, be- 

 cause the water is colder than the skin, and abstracts heat from it; but after a 

 time, when the temperature of the skin and water has become equalized, the 

 sensation ceases. If we let the hand rest for a short time in water at eight 

 degrees, which produces a feeling of cold, and then transfer it to water at six- 

 teen degrees R., we at first feel warmth, because the skin, whose temperature 

 has been considerably lowered by the first immersion, absorbs heat from the 

 warmer water ; but the feeling is soon transmuted into one of coldness, because 

 the skin, as soon as it is brought to a temperature of sixteen degrees, takes up 

 heat from the much warmer blood and gives this over in turn to the water. 

 Our thermometric sensations are, therefore, an uncertain criterion in judging of 

 the absolute temperature of external things ; and there are circumstances also 

 under which this uncertainty is increased. If we grasp, in winter, an iron rail, 

 it seems to us extraordinarily cold, much colder than the air, much colder than 

 a wooden rail under precisely the same circumstances ; and yet it is easily 

 shown by the thermometer that the three objects possess absolutely the same 

 temperature, and that hence the judgment founded on our sensations is false. 

 The ground of this illusion lies in the fact that iron is a very good, wood a 

 very bad, conductor of heat, whence the former robs our skin of its heat much 

 more rapidly than the latter. Since we have seen that it is the change of tem- 

 perature, not its absolute degree, which stimulates the nerves, it will readily be 

 conceived that a rapid change produces a more intense sensation than a gradual 

 one — that the feeling of cold is greater when the heat is withdrawn from the 

 skin in a relatively shorter time. Lastly, there is another circumstance to be 

 noticed which may be the occasion of deception in our estimate of temperature 

 through the sense of touch, a circumstance which conduces also to render our 

 estimate of weights uncertain, and exerts considerable influence on the intensity 

 of the sensation of pain. Just as two equal weights, when they press upon 

 surfaces of the skin of different extent, seem to us unequally heavy ; so does 

 the warmth or coldness of an external medium produce in us the impression of 

 a greater intensity in proportion to the greater extent of the surface on which 

 it takes efl^ect. If we dip into water at eight degrees of temperature, for in- 

 stance, the finger-end of one hand and the whole of the other hand, we feel in 

 the hand wholly immersed a much more intense feeling of cold than in the 

 finger-end. The ground of this error is probably that above given, that the 

 mind, from the greater number of nerves simultaneously excited, is betrayed 

 into the idea of a greater intensity of the sensation. 



As all thermomtJters are not equally sensitive, bnt the thickness of the walla 

 of the tube will in some cases prevent an equally rapid transmission of the ex- 

 ternal temperature and the consequent rise or fall of the quicksilver, so not 

 every part of our organ of touch is endued with the same sensibility to 

 changes of temperature in the surrounding medium ; and here, too, it is the 



