i68 SCIENCE AND THE HUMAN MIND 



calorimetry, or the measurement of a quantity of 

 heat. 



A similar theory, or rather two similar rival theories, 

 enabled Franklin, and his fellow- workers, to throw 

 light on the phenomena of electricity, which were 

 now drawing men's attention. The attractions and 

 repulsions between electrified bodies may be expressed 

 on the supposition that there is a substance, electricity, 

 which, like heat, may be treated as a quantity sub- 

 ject to the laws of addition and subtraction. With 

 electricity, however, the existence of two distinct and 

 opposite varieties was revealed in an early stage 

 of the history of its discovery. An electric charge 

 developed by friction on glass will neutralize a charge 

 produced in the same way on ebonite. These results 

 were explained by the supposition either of two fluids 

 with opposite properties, or of one fluid, of which the 

 excess or defect from the normal quantity gave rise 

 to the electrified state. The terms of speech appro- 

 priate to the one-fluid theory, with its positive and 

 negative electricities, are still with us, though, as we 

 shall see later, the fluid has given place to the newer 

 conception of electric particles or corpuscles, or to 

 the vaguer one of disembodied charges or electrons, 

 the excess of which constitutes the conventionally 

 negative, and the defect the conventionally positive, 

 electrification. 



The overthrow of the theory of phlogiston had the 



effect of bringing clearly to light the three states of 



The Atomic matter, solid, liquid and gaseous, as 



Theory. we know them now different physical 



phases of the same substance, which primarily may be 



