550 FOURTH REPORT — 1834. 



ciently small to pi'oduce rings of considerable diameter, other- 

 wise the surfaces in contact will not be sufficiently large to 

 allow sufficient effect to overcome the weight or inertia of the 

 glasses. 



The author has made various other researches on the sub- 

 ject, which have been recently communicated to the Royal So- 

 ciety, and which will appear in the forthcoming volume of its 

 Transactions. In particular he has attempted to compare the 

 times of communication of heat through two glasses in seve- 

 ral degrees of contact (as estimated by the tints), and has found 

 it rather more rapid with the higher degrees of contact. But 

 when the central l)lack is produced, it requires a considerable 

 heat to overcome the powerful attraction which subsists at that 

 minute distance. Some singular illustrations of the intensity 

 of this force have been observed. It seems not improbable that 

 at this distance there may be a limit where attraction becomes 

 predominant. The contact between glass and liquids is proba- 

 bly within this limit, since no heat seems capable of overcoming 

 the attraction. Again, the repulsive power seems capable of 

 being excited by heat only within a certain limit the other way, 

 and between surfaces regularly opposed to each other. An 

 iron at a bright red heat could not be made to repel a delicately 

 suspended gilt card disc, though brought to about one tenth 

 of an inch distance. 



Suggestions respecting Sir John HerscheVs Remarks on the 

 Theory of the Absorption of Lis^ht by coloured Media. By 

 the i?<?r. 'William Whewell, F.R.S. F.G.S. 



At the meeting of the British Association last year, Sir John 

 Herschel made some remarks*, the object of which was, I con- 

 ceive, to show, that though it may not be easy to determine at 

 present in what w-ay the dark lines of the spectrum and other 

 phaenomena of absorption are produced by the undulations of 

 the luminiferous aether, it is not difficult to show that there are 

 ways in which those undulations may produce phaenomena of 

 such a kind. I would beg leave to add one or two considera- 

 tions, which appear to me to bear upon what he then stated. 

 He observes that if undulations have to traverse canals which 

 ramify and meet again, there may be certain relations among 

 the lengths or other conditions of these canals which may pro- 

 duce a destruction of undulations for one particular rate of vi- 

 bration, and thus produce a dark line for one particular colour 

 in the luminiferous vibrations. To this view might be objected, 



• Since published in the P/iilosop/i/cal Mar/cizine, December, 183.3. 



