688 KANSAS CITY REVIEW OF SCIENCE. 



Two air-tight cylinders, A and B, contain air at ordinary pressure. The 

 cylinders are connected by a tube containing an air-pump, in such a way that 

 when the pump is worked, air is taken from A and forced into B. To use the 

 language of the electricians, we at once generate two kinds of pressure. The 

 vessels have acquired new properties. If we open a cock in the side of either 

 vessel, we hear a hissing sound. If a light body is placed before the opening in 

 A it would be attracted, and before the opening in B it would be repelled. Now 

 this is only roughly analagous to the case of the electrified bodies, but the analogy 

 will nevertheless aid us in our study. If the two vessels are first connected with 

 the air, and then closed up and the pump is set to work, we increase the pres- 

 sure in B and diminish the pressure in A. To do this requires the expenditure 

 of a quantity of work. If the cylinders are connected by an open tube — a con- 

 ductor — the difference in pressure disappears by reason of a flow of gas from one 

 vessel to the other. 



If we had a pump by means of which we could pump heat from one body 

 into another, starting with two bodies at the same temperature, the temperature 

 of one body would increase and that of the other would diminish. If we knew 

 less than we do of heat we might well discuss whether the plus sign should be 

 applied to the heat, or to the cold, because these names were coined by people 

 who knew very little about the subject except that these bodies produce different 

 sensations when they come in contact with the human body. 



Furthermore, we find that whether the hand is applied to a very hot body, 

 or to a very cold body, the physiological effect is the same. In each case the 

 tissue is destroyed and a burn is produced. Shall we now say that this burn is 

 produced by an unusual flow of heat from the hot body to the hand, or from the 

 hand to the cold body, or shall we say that it is due to an unusual flow of cold 

 from the cold body to the hand, or from the hand to the hot body ? 



Logically, these expressions are identical; still we have come to prefer one of 

 them. It is because we have learned that in those bodies which our fathers 

 called hot, the particles are vibrating Avith greater energy than in cold bodies, 

 that we prefer to'say that heat is added and not cold subtracted, when a cold 

 body becomes less cold. 



Now to'come back to our electrified bodies. Let us suppose that this gutta 

 percha, and this cat's-skin are not electrified. That means that their electrical 

 condition is the same as that of surrounding bodies. Let us also suppose that 

 their thermal condition is the same as surrounding bodies, ourselves included — 

 that is they are neither hot nor cold. We express these conditions in other words 

 by saying that the bodies have the same electrical potential, and the same tem- 

 perature. 



Temperature in heat is analagous to potential in electricity. As soon as 

 adjacent bodies are at different temperatures, we have the phenomena which 

 reveal to us the existence of heat. As soon as adjacent bodies have different 

 electrical potentials, we have the phenomena which reveal the existence of elec- 



