and the Economy of Fuel. 53 



Upon lifting up for an instant one side of the bottle, 

 and letting in a smaller quantity of cold air, the clouds 

 instantly returned, and continued circulating several 

 minutes with great rapidity, and then gradually disap- 

 peared as before. This experiment was repeated sev- 

 eral times, and always with the same result; the steam 

 always becoming visible when cold air was mixed with 

 it, and afterwards recovering its transparency when, part 

 of this air being expelled, that which remained had 

 acquired the temperature of the steam. 



Finding that cold air introduced under the bottle 

 caused the steam to be partially condensed, and clouds 

 to be formed, I was desirous of seeing what visible 

 effects would be produced by introducing a cold solid 

 body under the bottle. I imagined that if steam was a 

 conductor of heat, some part of the heat in the steam 

 passing out of it into the cold body, clouds would of 

 course be formed ; but I thought if steam was a non- 

 conductor of heat, that is to say, if one particle of 

 steam could not communicate any part of its heat to its 

 neighbouring particles, in that case, as the cold body 

 could only affect the particles of steam actually in con* 

 tact with it, no cloud would appear ; and the result of 

 the experiment showed that steam is in fact a non-con- 

 ductor of heat. For, notwithstanding the cold body used 

 in this experiment was very large and very cold, being a 

 solid lump of ice nearly as large as a hen's egg, placed 

 in the middle of the hollow cavity under the bottle, upon 

 a small tripod or stand made of iron wire ; yet as soon 

 as the clouds which were formed in consequence of the 

 unavoidable introduction of cold air in lifting up the 

 bottle to introduce the ice were dissipated, which soon 

 happened, the steam became so perfectly transparent 



