508 Prof. Tyndall's Contributions to Molecular Physics. 

 basalt. I observed 



10 0.201 =3U° (30° 15'calc.) 

 110 (10 0) 110 = 59-61° (60 „ ) 



The plane 101 seems also to occur on some of these crystals. 

 The plane 01 on one of them showed re-entering angles, 

 which, although not measurable, are also a proof of the twin 

 structure. 



A very interesting specimen of Herschelite from Victoria, 

 Australia, was lately given to the British Museum by Mr. 

 Selwyn, the colonial geologist, who found it himself. The crys- 

 tals, occurring, like those just described, on basalt, are aggre- 

 gated together in a greater quantity ; and although much larger 

 than those from Italy, are still less fit for good measurements, the 

 planes being broken in every direction. They have the form of 

 fig. 1 ; and I observed on one of the crystals 



5 2.502 =50i° (50° 0' calc). 



The angles of the top plane are very much rounded, so that the 

 edges 5 2.502 are quite obliterated. 



The optical properties of these crystals are exactly the same as 

 those of the Italian specimens. 



LXIII. The Bakerian Lecture. — Contributions to Molecular 

 Physics. Being the Fifth Memoir of Researches on Radiant 

 Heat. By John Tyndall, F.R.S., <Sfc. 



[Continued from p. 458.] 



§ VII. Influence of temperature on the transmission of radiant 



heat. 



^^HE power of varying at will the temperature of the platinum 

 spiral renders it peculiarly suitable for the examination 

 of the influence of temperature on the transmission of radiant 

 heat. To obtain sources of different temperatures, Melloni 

 resorted to lamps, to spirals heated to incandescence by the 

 flame of alcohol, to copper laminae heated by flame, and to the 

 surfaces of vessels containing boiling water. No conclusions 

 regarding temperature can, as will afterwards be shown, be 

 drawn from such experiments ; but by means of the platinum 

 spiral we can go through all those changes of temperature, 

 retaining throughout the same vibrating atoms, and we can there- 

 fore investigate how the alteration of the rate of vibration affects 

 the rate of absorption. The following series of experiments were 

 executed on the 9th of October, with a platinum spiral raised to 

 barely visible redness, and vapours at a tension of 0*5 of an inch. 



