206 Historical Review of Experiments 



one, and whether I pursued it with zeal and persever- 

 ance. 



The few remarks and observations which follow were 

 occasioned by my researches made at that time.* 



All the different substances which I had yet made use 

 of for covering the bulb of the thermometer (which 

 was contained within a glass globe an inch and a half 

 in diameter) had in a greater or less degree confined 

 the heat and prevented it from passing into or out 

 of the bulb of the thermometer as rapidly as it would 

 otherwise have done. Here then arose the important, 

 and as yet unanswered question, how and by what 

 mechanical operation had the coverings in question pro- 

 duced these effects ? 



This much is certain, that the slowness of the cooling 

 of the bulb of the thermometer cannot by any possibil- 

 ity be a result of the non-conducting powers of those 

 substances of which the coverings consisted, consid- 

 ered simply as having hindered the passage of the heat, 

 for if, instead of regarding them merely as bad conduc- 

 tors of heat, we were to suppose them to have been 

 totally impervious to heat, still their volumes that 

 is, the sum of all their solid parts or fibres would 

 be so inconsiderable in proportion to the space they 

 occupied, that they would either have produced no 

 effect on the air filling their interstices, or this air would 

 have been sufficient of and for itself to have conducted 

 all the heat communicated in less time than was actually 

 taken up in the experiments. Here is the proof of this 

 statement. 



The diameter of the glass globe being 1.6 inches, its 

 contents amounted to 2.14466 cubic inches. The di- 



* See my eighth Essay, Vol. I. p. 455. 



