168 CHEMISTRY. 



considerations, and particularly from this, that a 

 thermometer placed in vacuo will grow warm almost 

 as soon as one not in vacuo. 



Extreme Tenuity. 



The thinnest substance ever observed is the aque- 

 ous film of the soap bubble previous to its bursting; 

 yet it is capable of reflecting a faint image of a can- 

 dle or of the sun. Hence its thickness must cor- 

 respond with what Sir Isaac Newton calls the begin- 

 ning of black, which appears in water at a thickness 

 of the -Tjfl^nrth P art f an inch. 



Reflected Heat. 



Buffon kindled wood by reflection from mirrors, 

 at the distance qf 210 feet. 



A drop of water rolls about on a red-hot iron 

 without evaporation, because its surface becomes so 

 highly polished as to reflect all the heat. If the heat 

 be less, the water penetrates the pores of the oxi- 

 dated iron, and losing its polish, is evaporated. 



Heat Conductors. 



When diiferent bodies are exposed to the same 

 source of heat, they suffer it to pass through them 

 with different degrees of velocity ; or, they have 

 various conducting powers with regard to heat. 

 Good conductors, when touched, occasion a greater 

 sensation of heat and cold than bad ones. 



When the body feels cold, the caloric is passing 

 out of it into the neighbouring object; when the 

 body feels warm, it is receiving heat. 



Motes in the Sun-beam. 



If the temperature of a metallic stove for heating 

 a room be raised much higher than 300 Fahrenheit, 

 the animal and vegetable matter, which is found me- 



