POPULAK FALLACIES. 



theless, confined within narrow and vague limits in most of its 

 indications. 



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, differently illuminated, we shall in 

 the same way be in some cases able to declare which is the more 

 splendid ; but if their splendour 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 temperature, the touch will sometimes inform us 

 which is the hotter ; but if they be nearly equal, we shall be un- 

 able to decide which has the greater or which the less temperature. 



The sense of touch, however, totally fails in informing us of 

 the comparative quantities of heat in bodies. It cannot be at all 

 affected by tha.t part of the heat of a body which is latent. 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 considerable quantity of water. But it 

 is not only in the case of latent heat, which cannot be felt at all, 

 that the touch fails to inform us of the quantities of heat in a 

 body. Different bodies are raised to the same temperature 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 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 communicated 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 

 different 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. 



The thermometer, the scientific measure of temperature, is 

 here, however, in the same predicament as the sense of feeling, 



