Sect. XIV/6. OF IDEAS. 91 



tion of heat eflentially differs from the perceptions of the fenfe 

 of touch, as we receive pain from too great preflure of folid 

 bodies, but none from the abfence of it. It is hence probable, 

 that nature has provided us with a fet of nerves for the percep- 

 tion of this fluid, which anatomifts have not yet attended to. 



There may be fome difficulty in the proof of this afTertion ; 

 if we look at a hot fire, we experience no pain of the optic 

 nerve, though the heat along with the light muft be concentra- 

 ted upon it. Nor does warm water or warm oil poured into 

 the ear give pain to the organ of hearing ; and hence as thefe 

 organs of fenfe do not perceive fmall excefTes or deficiencies of 

 heat ; and as heat has no greater analogy to the folidity or to 

 the figures of bodies, than it has to their colours or vibrations ; 

 there feems no fufficient reafon for our afcribing the perception 

 of heat and cold to the fenfe of touch 5 to which it has gener- 

 ally been attributed, either becaufe it is diffufed beneath the 

 whole fkin like the fenfe of touch, or owing to the inaccuracy of 

 our obfervations, or the defect of our languages. 



There is another circumftance would induce us to believe, 

 that the perceptions of heat and cold do not belong to the or- 

 gan of touch ; fince the teeth, which are the leaft adapted for 

 the perceptions of folidity of figure, are the moft fenfible to heat or 

 cold ; whence we are forewarned from fwallowing thofe mate- 

 rials, whofe degree of coldnefs or of heat would injure our ftom- 

 achs. 



The following is an extract from a letter of Dr. R. W. Dar- 

 win, of Shrewfbury, when he was a ftudent at Edinburgh. " I 

 made an experiment yefterday in our hofpital, which much fa- 

 vours your opinion, that the fenfation of heat and of touch de- 

 pend on different fets of nerves. A man who had lately recov- 

 ered from a fever, and was ftill weak, was feized with violent 

 cramps in his legs and feet ; which were removed by opiates, 

 except that one of his feet remained infenfible. Mr. Ewart 

 pricked him with a pin in five or fix places, and the patient de- 

 clared he did not feel it in the leaft, nor was he fenfible of a very 

 fmart pinch. I then held a red-hot poker at fome diftance, and 

 brought it gradually nearer till it came within three inches, 

 when he aflerted that he felt it quite diftinclly. I fuppofe fome 

 violent irritation on the nerves of touch had caufed the cramps, 

 and had left them paralytic ; while the nerves of heat, having 

 fuffered no increafed flimulus, retained their irritability." 



Add to this, that the lungs, though eafily ftimulated into in- 

 flammation, are not fenfible to heat. See Clafs III. 1. 1. 10. 



VII. Of 



