Prof. Tyndall's Contributions to Molecular Physics. 515 



Even when the spiral was at a barely visible heat, the iodide was 

 decidedly the most opake of the two ; the same result was obtained 

 with a spiral heated below redness, as proved by the following 

 figures : — 



Name of vapour. 



Deflection. 



Absorption. 



Chloroform 



o 



8-5 



12-14 



Chloroform . . 



8-5 



12-14 



Iodide of methyle 



. 100 



14-28 



Iodide of methyle 



. 10-0 



14-28 



Total heat . . . 



47-3 



100-0 



Here the iodide is still predominant. Is it, then, a question 

 of temperature merely ? or is there a special flux emitted by the 

 lampblack, to which chloroform is particularly opake ? In other 

 words, is there a special accord between the rates of vibration of 

 lampblack and chloroform ? To answer this question I operated 

 thus : — The platinum spiral was heated by only two cells, and 

 the strength of this current was lowered by the introduction of 

 resistance. When decidedly below a red heat, the spiral was 

 plunged into boiling water. Bubbles of steam issued from it, 

 proving that its temperature was above 212° Fahr. By augment- 

 ing the resistance its heat was lowered, until it was no longer 

 competent to produce the least ebullition. It was then with- 

 drawn from the water, and employed as a source : the following 

 are the results : — 



Table XXII. — Source, platinum spiral at 100° C. 



Name of vapour. Deflection. Absorption. 



Bisulphide of carbon . . 5*7 7*03 



Chloroform 14*0 16-8 



Iodide of methyle . . . 15-3 . . 18-0 



No reversal was here obtained. The temperature was then 

 reduced so that the total heat fell from 81 units to 59 units ; but 

 not even in this case (when the temperature was considerably 

 below that of boiling water) could the reversal be obtained. The 

 absorptions approach each other, but the iodide has still the ad- 

 vantage of the chloroform. 



Table XXIII. — Source, platinum spiral, heated under 100° C. 



Name of vapour. Deflection. Absorption. 



o 



Bisulphide of carbon . . 5*2 9 # 2 



Chloroform 10'0 17'3 



Iodide of methyle . . . 10-8 18*2 



It is not, therefore, temperature alone which determines the io- 



