136 SENSE OF TOUCH. 



One of the great purposes of the sense of tact is to enable us to 

 judge of the temperature of bodies. This office it executes alone. No 

 other sense participates in it. It requires no previous exercise ; is felt 

 equally by the infant and the adult, and requires only the proper de- 

 velopment of its organs. The relative temperature of bodies is accu- 

 rately designated by the instrument called the thermometer; but very 

 inaccurately by our own sensations; and the reason of this inaccuracy 

 is sufficiently intelligible. In both cases, the effect is produced by the 

 disengagement of a subtile fluid, called caloric or the matter of heat, 

 which pervades all bodies, and is contained in them to a greater or less 

 extent. This caloric is constantly passing and repassing between bodies, 

 either by radiation or by conduction, until there is an equilibrium of 

 caloric and all have the same temperature as indicated by the ther- 

 mometer. Hence, objects in the same apartment will exhibit, cseteris 

 paribus, a like temperature by this test. From this law, however, the 

 animal body must be excepted. The power which it possesses of gene- 

 rating its own heat, and of counteracting the external influences of 

 temperature, preserves it constantly at the same point. 



Although, however, all objects may exhibit the same temperature, 

 in the same apartment, when the thermometer is applied to them ; the 

 sensations communicated by them may be very different. Hence the 

 difficulty, which the uninstructed have in believing, that they are actually 

 of identical temperature ; that a hearth-stone, for instance, is of the 

 same degree of heat as the carpet in the same chamber. The cause of 

 the different sensations experienced in the two cases is, that the hearth- 

 stone is a much better conductor of the matter of heat than the carpet. 

 The consequence is, that caloric is more rapidly abstracted by it from 

 the part of the body which comes in contact with it, and the stone 

 appears to be the colder of the two. For the same reason, when these 

 two substances are raised in temperature above that of the human body, 

 the hearth-stone will appear the hotter of the two ; because, it conducts 

 caloric and communicates it more rapidly to the body than the carpet. 



When the temperature of the surrounding air is higher than 98, we 

 receive caloric from the atmosphere, and experience the sensation of 

 heat. The human body is capable of being penetrated by the caloric 

 of substances exterior to it, precisely like those substances themselves ; 

 but, within certain limits, it possesses the faculty of consuming the heat, 

 and retaining the same temperature. When the temperature of the 

 atmosphere is only as high as our own an elevation which it not un- 

 frequently attains in many parts of the United States we still expe- 

 rience the sensation of unusual warmth ; yet no caloric is communicated 

 to us. The cause of this feeling is, that we are accustomed to live in 

 a medium of a less elevated temperature, and consequently to give off 

 caloric habitually to the atmosphere. 



Lastly, in an atmosphere of a temperature much lower than that of 

 the body, heat is incessantly abstracted from us ; and, if rapidly, we 

 have the sensation of cold. From registers, kept by the illustrious 

 founder of the University of Virginia, Mr. Jefferson, at his residence 

 at Monticello, 1 lat. 37 58', long. 78 40', it appears that the 



1 Virginia Literary Museum, p. 36, Charlottesville, 1830. 



