80 



Heat seems to be nothing but motion : but this mo- 

 tion has some peculiar circumstances. 1. It is expan- 

 sive motion, whereby a body endeavours to dilate itself." 

 2. This motion is upward, and toward the circumfer- 

 ence. 3. It is not an equable motion of the whole, but 

 oniy of the smaller particles of the body. 4. It is a rapid 

 jnotiosi. Heat may therefore be defined, an expansive 

 undulatory motion in the minute particles pf a body; 

 whereby they rapidly tend to the circumference, and at 

 the same time upward. 



Fire has some effect on most bodies, even in an ex* 

 hausted receiver. One placed a black ribbon therein, 

 and then applied a burning-glass. Abundance of smoke 

 issued out of it, which fell by Sittle and little, and the 

 ribbon appeared not at all changed. But when it was 

 touched, after the re-admission of the air, it presently, 

 fell into ashes. 



The glass being applied to gun-powder so inclosed, i 

 burnt grain by grain, but none of the grains kindled 

 Another time when the sun had less force, they woulc 

 not burn, but only boiled and emitted smoke. Thi 

 smoke falling on the board on which the power lay, wa 

 of the colour of brimstone. Th powder that re 

 mained being put on coals, burned like salt-petre, iua's 

 much as the brimstone had exhaled. 



Tin and copper melted together weigh more than 

 both bodies did before. Yea, orpin being mixed wit 

 salt. of tartar, is heavier by a fifth part. 



To account for this, it has been commonly supposed 

 that fire adds to the weight of bodies. But fire lias il 

 self lie weight at all : therefore it can give none. Pur 

 iire, as Dr. Hillary observes, is a body without gravity 

 and has no more tendency to any one part of space, tha 

 to any other. 



Is not then this alteration of weight rather owing t 

 an alteration of the inward texture of the particles in 

 the body calcined ? The lighter particles being re- 

 moved by exhalation, do not those remaining ap- 

 proach nearer e*ch other! And must not then the! 



