1826.] Rev. Mr, Ritchie^ Reply to the Rev, Mr, Powell 123 



Philosophical Journal, I consider myself bound, in justice to thQ 

 cause of science, to answer his objections. This will be best 

 accomplished by a single deduction from the following 



Experiment, 



. Let two air thermometers be procured having their bulb^ 

 large, and blown extremely thin (this condition is absolutely 

 necessary to the success of the experiment) with scales divided 

 into any number of equal parts. Place these at a convenient 

 distance from each other, and then place a heated iron ball 

 between them in such a position that the fluids in the two stem^ 

 will sink exactly the same number of degrees. Let one of the 

 hemispheres in the ball A, formed by a plane passing through 

 the centres of the two balls, be coated with china ink. Let two 

 of the alternate quarters of the ball jB, formed by a plane cutting 

 the former at right angles, be also coated with china ink. Place 

 the thermometers in their original position, raise the iron ball 

 to an elevated temperature (though still invisible in the dark), 

 place it in its former position, and carefully observe the number 

 of degrees the fluid descends in each stem. A striking difFe^^ 

 ence will now be observed. The fluid will be found to have 

 sunk several degrees lower in the thermometer B than in A. 

 The same experiment may also be performed with a differential 

 thermometer, having the bulbs coated as formerly described. 



Whatever be the cause of this striking difference, it cannot pos- 

 sibly be the one assigned by Mr. Powell. For the surfaces of 

 the two balls having exactly the same quantity of coating must 

 radiate the absorbed heat with equal rapidity. I have viewed 

 the subject in every way I could think of, and can find no cause 

 adequate to produce this striking difference, except the one 

 which I formerly assigned ; viz. that the portion of caloric 

 which radiated freely through the transparent hemisphere in one 

 of the balls, was interrupted by the opposite posterior coating on 

 the other ball. 



With regard to the experiments with coated and transparent 

 screens, I would only remark that a common mercurial thermo- 

 meter is quite inadequate to determine the fact. If a quantity 

 of water at the temperature of 50° be mixed with a hundred times 

 its bulk of water heated to the temperature of 50-1-°, the common 

 thermometer will not detect the difference. In like manner, if 

 the quantity of heat which freely permeates a thin plate of glass, 

 amount only, in peculiar circumstances, to -^ or -^^ of a 

 degree of Fahrenheit's thermometer, the common mercurial 

 thermometer can not possibly determine its existence. I would, 

 therefore, humbly recommend to Mr. Powell to procure screens 

 of extreme tenuity, and repeat the experiments with a more deli- 

 cate instrument than a common therinometer, and he will 

 assuredly find that the results which I have stated are not hasty 



