Prof. Tyndall on some Physical Properties of Ice. 333 



which is always present, to buru off first, then the colours of the 

 other substances become distinctly visible. 



For this mode of distinguishing the alkalies and alkaline 

 earths, very small quantities of substance are necessary ; and the 

 analyses can be made with somewhat better results when day- 

 light is excluded, 



I am indebted to Professor Kirchhoff for determinations of 

 the rays which come through the solutions of indigo and the 

 blue glass that I have used in my experiments. The dilute so- 

 lution of indigo allows all the rays of the spectrum to pass except 

 the dull red ones bordering on the orange. The solution of 

 indigo used to distinguish the alkalies, allows those rays to pass 

 extending from A to B and from E to G ; and the blue glass 

 those extending from A to a, and from F to H of Fraunhofer's 

 lines of the spectrum. 



A little reflection will show that the method described in this 

 paper depends on the following principle. If there be any dif- 

 ference in colour between two flames, the medium which will 

 give white light with one of the flames will give coloured light 

 with the other. Two substances, therefore, which communicate 

 colours to the flame, unless these colours be exactly alike, can 

 be distinguished by a medium producing white light with only 

 one of them. 



By following out the investigation I hope to be able to deter- 

 mine potash quantitatively in the presence of soda, and to effect 

 separations between some of the heavy metals. 



I am indebted to Professor Bunsen, in whose laboratory this 

 investigation was carried on, for many valuable suggestions. 



Heidelberg, September 1858. 



XXXVII. On some Physical Properties of Ice. By John Tyn- 

 dall, F.R.S., Professor of Natural Philosophy in the Royal 

 Institution of Great Britain^. 



THE observations upon Glaciers, to be recorded in a subse- 

 quent paper, led me to make some experiments upon the 

 physical properties of ice, the results of which are, I hope, of 

 sufficient interest to justify me in presenting them to the Royal 

 Society. 



§1. 



1. I availed myself of the fine sunny weather with which we 

 were favoured last September and October, to examine the effects 



* From the Philosophiciil Transactions, Part I. for 1858 ; having been 

 rea«l at the Royal Society December 17, 1857. 



