480 Prof. Tyndall on the Action of Free Molecules on 



The range of specific gravity of the crystallised mineral is 

 a little wider than that of the massive as determined by Breit- 

 haupt : one crystal presenting no external sign of impurity 

 was found to have a specific gravity of 6'48 ; a second had a 

 specific gravity of 6*37, but, like the less dense specimens of 

 Breithaupt, evidently contained foreign matter. Two other 

 fairly large crystals had specific gravities of 6'56 and (v72 

 respectively; while a small apparently very pure crystal, with 

 smooth lustrous faces, gave a result as high as 6*86. 



LXIII. Action of Free Molecules on Radiant Heat, and 

 its Conversion thereby into Sound. By John Tyndall, 

 F.RS, 



[Concluded, from p. 462.] 



Contents. 



Page 

 Sect. 5. New Experiments. Hypothesis of Internal Films , . 480 



a. The Experimental Tube 480 



b. The Plates of Rock-salt 486 



6. Conservation of Molecular Action 491 



7. Thermal Continuity of Liquids and Vapours 495 



8. Rhythmic Absorption of Radiant Heat by Gases and 



"Vapours 500 



9. Manornetric Measurements 508 



10. Application of Results to Meteorology 514 



§ 5. New Experiments. Hypothesis of Internal Films. 



a. The Experimental Tube. 

 WISH now to come to closer quarters with the argument 



I 



urged by Magnus and repeated by MM. Lecher and 

 Pernter, namely that my results were due to " vapour- 

 hesion " — that is to say, to liquid films condensed on my ex- 

 perimental tube and on my plates of rock-salt. The two inves- 

 tigators last named express unqualified surprise that I should 

 have neglected the simple precaution of experimenting with 

 blackened tubes. But this precaution was by no means neg- 

 lected by me. I have repeatedly fortified myself by experi- 

 ments of this character, with the result recorded in Table I. 

 But I went further. A smooth coating of lampblack, how- 

 ever powerful as an absorber, might be competent to reflect a 

 certain portion of the incident heat. Hence my desire to get 

 entirely rid of reflection, by avoiding all contact with the 

 interior surface of the experimental tube. 



In fig. 3 an apparatus is sketched, by means of which this 

 has been accomplished. T T' is a stout tube of brass, 36 

 inches long and 6 inches internal diameter. Projecting from 



