86 



POPULAR FALLACIES. 



bility, be utterly useless in the hands of the merchant or the housewife. Each 

 class of instruments has, however, its peculiar uses ; and is adapted to give 

 indications with that degree of accuracy which is necessary and sufficient for 

 the purpose to which it is applied. 



The term heat in its ordinary acceptation, is used to express a feeling or 

 sensation which is produced in us when we touch a hot body. We say that 

 the heat of a body is more or less intense, according to the degree in which 

 the feeling or sensation is produced in us. The term is often, however, 

 used in a somewhat different sense. It is here applied to express a cer- 

 tain state of body, which is attended with certain distinct mechanical effects, 

 many of which are capable of being actually measured, and one of which only 

 is the effect produced on our organs, and through them, on the mind, to which 

 alone, in the popular sense, the term heat is applied. This distinction in the 

 use of the term has induced some philosophers to adopt another word, caloric, 

 to express the physical effect, while the common term, heat, has been retained 

 to express the sensation. It does not appear to us to be necessary to adopt this 

 term, because it never happens that any confusion arises from the two senses 

 of the term heat ; and, besides, the use of the term caloric is apt to lead the 

 mind to the assumption of an hypothesis, or theory, concerning the nature of 

 heat, the consequences of which are apt to be mixed with that investigation 

 which should be founded on the results of experiment alone. 



The touch, by which we acquire the perception of heat, like the eye, ear, 

 and other organs, is endowed with a sensibility confined within certain limits ; 

 and even within these we do not possess any exact power of perceiving or 

 measuring the degree of the quality by which the sense is affected. If we 

 take two heavy bodies in the hand, we shall in many cases be able to declare 

 that one is heavier than the other ; but if we are asked whether one be exactly 

 twice as heavy, or thrice as heavy as the other, we shall be utterly unable to 

 decide. In like manner, if the weights be nearly equal, we shall be unable to 

 declare whether they are exactly equal or not. If we look at two objects, differ- 

 ently illuminated, we shall in the same way be in some cases able to declare 

 which is the more splendid ; but if their splendor be nearly equal, the eye 

 will be incapable of determining whether the equality of illumination be exact 

 or not. It is the same with heat. If two bodies be very different in tempera- 

 ture, the touch will sometimes inform us which is the hotter ; but if they be 

 nearly equal, we shall be unable to decide which has the greater or which 

 the less temperature. 



But even this information, rude and unsatisfactory as it is, is more full than 

 that which the evidence of the touch frequently furnishes. 



After what has been explained in the preceding part of this treatise, the 

 reader will have no difficulty in perceiving that feeling can never inform us of 

 the quantity of heat which a body contains, much less of the relative quantities 

 contained in two bodies. In the first place, the touch can never be affected by 

 heat which exists in the latent state. Ice-cold water, and ice itself, feel to have 

 the same temperature, and to contain the same quantity of heat ; and yet it 

 is proved that ice-cold water contains a great deal more heat than ice ; nay, 

 that it can be compelled to part with its redundant heat, and to become ice ; and 

 that this redundant heat, when so dismissed, may be made to boil a considera- 

 ble quantity of water. But it is not only in the case of latent heat, which can- 

 not be felt at all, that the touch fails to inform us of the quantities of heat in a 

 body. It has been shown that different bodies are raised to the same tempera- 

 ture by very different quantities of heat. If water and mercury, both at the 

 temperature of 32°, be touched, they will be felt to be both equally cold ; and 

 \ if they be both raised to 100° and then touched, they will be felt to be both 



