POPULAR FALLACIES. 



87 



equally warm ; and the inference would be, that equal quantities of heat must 

 have been in the meanwhile communicated to them. Now, on the contrary, it 

 has been proved that, in this case, the quantity of heat which has been com- 

 municated to the water is not less than thirty times the quantity which has been 

 imparted to the mercury. In fact, to cause the same change of temperature, 

 and, therefore, the same feeling of heat, in different bodies, requires very differ- 

 ent quantities of heat to be imparted to them. It is plain, therefore, that the 

 sense of touch totally fails in the discovery of the quantities of heat which 

 must be added to different bodies, in order to produce in them the same change 

 of temperature. 



But it may be said that the thermometer itself is here in the same predica- 

 ment as the touch, and that this scientific measure of heat likewise fails to in- 

 dicate the quantity of that principle which has been added or subtracted. Set- 

 ting aside, however, the estimation of quantities of heat, the sense of touch is 

 not less fallacious in the indications which it gives of temperature itself ; and 

 here, indeed, the error and confusion into which it is apt to lead, when unaided 

 by the results of science, are very conspicuous. If we hold the hand in wa- 

 ter which has a temperature of about 90°, after the agitation of the liquid has 

 ceased we shall become wholly insensible of its presence, and will be uncon- 

 scious that the hand is in contact with any body whatever. We shall, of course, 

 be altogether unconscious of the temperature of the water. Having held both 

 hands in this water, let us now remove the one to water at a temperature of 

 200°, and the other to water at the temperature of 32°. After holding the 

 hands for sometime in this manner, let them be both removed, and again im- 

 mersed in the water at 90° ; immediately we shall become sensible of warmth 

 in the one hand, and cold in the other. To the hand which had been immersed 

 in the cold water, the water at 90° will feel hot, and to the hand which had 

 been immersed in the water at 200°, the water at 90° will feel cold. If, there- 

 fore, the touch be in this case taken as the evidence of temperature, the same 

 water will be judged to be hot and cold at the same time. 



If, in the heat of summer, we descend into a cave, we become sensible that 

 we are surrounded by a cold atmosphere ; but if, in the rigor of a frosty win- 

 ter, we descend into the same cave, we are conscious of the presence of a 

 warm atmosphere. Now a thermometer suspended in the cave on each of these 

 occasions, will show exactly the same temperature, and, in fact, the air of the 

 cave maintains the same temperature at all seasons of the year. The body, how- 

 ever, being in the one case removed from a warm atmosphere into a colder one, 

 and in the other case, from a very cold atmosphere into one of a higher tempera- 

 ture, becomes in the latter case sensible of warmth, and in the former, of cold. 



Thus we see that the sensation of heat depends as much on the state of our 

 own bodies, as that of the external bodies which excite the sensation ; the 

 same body at the same temperature producing different sensations of heat and 

 cold, according to the previous state of our bodies when exposed to it. 



But even when the state of our bodies is the same, and the temperature of 

 external objects the same, different objects will feel to us to have different de- 

 grees of heat. If we immerse the naked body in a bath of water at the tem- 

 perature of 120°, and, after remaining for some time immersed, pass into a room 

 in which the air and every object is raised to the same temperature, we shall 

 experience, in passing from the water into the air, a sensation of coldness. If 

 we touch different objects in the room, all of which are at the temperature of 

 120°, we shall nevertheless acquire very different perceptions of heat. When ! 

 the naked foot rests on a mat or carpet, a sense of gentle warmth is felt ; but i 

 if it be removed to the tiles of the floor, heat is felt sufficient to produce incon- ] 

 venience. If the hand be laid on a marble chimney-piece, a strong heat is < 



