1867.] Physics. 431 



promises great success. The current is said to be able to deposit a 

 pound of copper per hour. 



Mr. Mellor, manager of the Magnesium Metal Company, has 

 been experimenting on the alloys of Thallium and Magnesium, with 

 the object of seeing whether, if such an alloy could be easily made 

 into wire, it might be found to burn readily and to produce an intense 

 bright green flame, when, from the portability of the wire, it would 

 be well adapted to some of the purposes for which a green flame is 

 required. It was found that thallium alloys most readily with 

 magnesium, and in any proportions. The alloys are very stable, 

 and are easily worked up into wire and ribbon. Alloys containing 

 5, 10, 15, 20, 25, and 50 per cent, of thallium were prepared; 

 these all bum brightly and steadily, but the flame is smaller and 

 the combustion slower than that of pure magnesium. The heat- 

 conducting property of the alloy compared with magnesium is sen- 

 sibly diminished, showing the change in the molecular construction 

 of the metal. The smoke produced in the combustion of these 

 alloys is more dense, and as it curls gracefully away it is seen to be 

 fringed with a rather pretty dark-purple tint ; but the magnesium 

 light is so very intense, that it almost completely masks the thal- 

 lium flame, so that it is not observable in some of the alloys — 

 indeed, the green light is scarcely recognizable even in an alloy con- 

 taining 50 per cent, of thallium. 



As a concluding paragraph to our Chronicles of Optics we give 

 a very excellent method of cleaning glass, which may be useful 

 where other methods fail. It is taken from the Appendix of the 

 second edition of Major Eussell's ' Tannin Progress,' published by 

 Eobert Hardwicke, Piccadilly. Dilute the ordinary hydrofluoric 

 acid, sold in gutta-percha bottles, with four or five parts of water ; 

 drop it on a cotton rubber (not on the glass), and rub well over; 

 afterwards washing till the acid is removed. The action is the same 

 as that of sulphuric acid when used for cleaning copper ; a little of 

 the glass is dissolved off, and a fresh surface exposed. The solution 

 of the acid in water does not leave a dead surface on the glass as the 

 vapour would ; if a strong solution is left on long enough to pro- 

 duce a visible depression, the part affected will be quite bright. 

 This method is recommended in some cases for cleaning photo- 

 graphic plates ; but we should think it might also be useful in 

 cleaning the insides of bottles, flasks, &c, which have got stained 

 through use. 



Heat. — M. Fizeau has been examining the abnormal contrac- 

 tion and dilatation of iodide of silver under the influence of heat. 

 He finds that in this salt, whether in its amorphous or crystalline 

 state, the action of heat is reversed ; its dilatation being negative, 



