270 Prof. Tyndall on the Absorption and 



For the case when .r = or A' coincides with 0, when a~2b, 

 we nnd/l+^y\ = 1*64792, or 7i='32396 (l + y)nearly; 



and then if 



a = 2b, the roots are x = -f-#, and x = 0; 



a = 6b, „ x = a, „ x — —'717a; 



a = 9b, „ x= a, „ x=— *978a. 



With respect to my own experiments, I have found a note 

 that the tube of the barometer-gauge passed to ^ inch below the 

 surface of the mercury, and that there was a depth of '85 inch 

 of mercury below it in the cistern ; but the radius of the tube 

 of the gauge is not stated ; and even if the value of the dynamical 

 adhesion of the mercury to the glass were known, the value of n 

 would remain unknown. 



The measures on which the calculations for the degree of cold 

 produced, from Amontons's formula, were made in my paper of 



January 1862, were as follows : — 



o 



When a=201 in., the mercury rose to 2*64 in., and the calc. cold =11*59 ; 

 a=6-20 „ „ 7-60 „ „ =29-4; 



a=6-\8 „ „ 8-18 „ „ =43-2; 



a=9-20 „ „ 11-25 „ „ =50'5. 



With b so small as ^ inch, the small extent of the oscillations 

 here shown proves that the adhesion must have been much 

 greater than supposed in any of the preceding computations, 

 even if no cold arose from the rarefaction. 



The problem is evidently a dynamical one before it becomes a 

 thermometrical one ; and if we apologize for the chemists who 

 invented or employed the experiment, what must we say for the 

 mathematicians who have accepted it as a purely thermometric 

 experiment ? 



XXXVI. On the Absorption and Radiation of Heat by Gaseous 

 Matter. — Second Memoir. By John Tyndall, F.R.S., 

 Professor of Natural Philosophy in the Royal Institution*. 



§ 1. JNSTRUMENTS.— The apparatus made use of in this 

 inquiry is the same in principle as that employed in my 

 last investigation j-. It grew up in the following way : — A tube 

 was first procured to receive the gases through which radiant heat 

 was to be transmitted ; but it was necessary to close the ends of 



* From the Philosophical Transactions, Part I. for 1862, having been 

 read at the Royal Society January 30, 1862. 

 f Philosophical Transactions, 1.861; and Phil. Mag, voLxxii. p. 169. 



