402 



Profs. G. D. Liveing and J. Dewar. [June 16, 



vibrated, not during, but after, the changes of load through which the 

 given state has been reached. 



It is also shown that this approximate equality produced by vibra- 

 tion continues after the vibration ceases. Also, that when a cycle of 

 loads is gone through afterwards, without vibration, the old difference 

 between the " on " and " ofE " curves reasserts itself. 



It is suggested that the cyclic phenomenon so conspicuous in this 

 investigation is not peculiar to the thermoelectric effects of stress, but 

 is probably present in other effects of stress, and may perhaps be found 

 to occur in the changes of any quality of matter which is a function of 

 another variable quality (such as temperature) when the latter quality 

 is subjected to increment and decrement.' 



Lastly the results of certain independent experiments made by others 

 in other branches of physics are referred to in confirmation of this 

 suggestion. 



XII. "On the Reversal of the Lines of Metallic Vapours. No. 

 VIII. (Iron, Titanium, Chromium, and Aluminium.)" By 

 G. D. Liveing, M.A., F.R.S., Professor of Chemistry, and J. 

 Dewar, M.A., F.R.S., Jacksonian Professor, University of 

 Cambridge. Received June 2, 1881. 



In our last communication on this subject we observed (" Proc. 

 Roy. Soc," vol. 29, p. 405) that iron introduced as metal, or as 

 chloride, into the electric arc, in a lime crucible, in the way which had 

 proved successful in the case of many other metals, gave us no reversals. 

 We succeeded, however, in reversing some ten of the brightest lines of 

 iron, mostly in the blue and violet, by passing an iron wire through 

 one of the carbons, so as to keep up a constant supply of iron in the 

 arc. Considering the great number of iron lines, and that so many of 

 them are strongly represented amongst the Fraunhofer lines, it seemed 

 somewhat surprising that it should be difficult to obtain a reversing 

 layer of iron vapour in the arc inclosed as we use it in an intensely 

 heated crucible. A like remark might be made respecting titanium, 

 which is almost as well represented as iron in the Fraunhofer lines, 

 but has heretofore given us no reversals. Almost the same might be 

 said of chromium, except that the number of chromium lines is so 

 much less than that of either of the other two metals. 



We have since found that most, if not all, of the strong lines of 

 these three metals may be reversed by proper management of the 

 atmosphere and supply of metal in the crucible. Indeed, with regard 

 to iron we have found that the method employed with other metals 

 was successful so far as the ultra-violet rays were concerned, though it 



