1879.] On the Reversal of the Lines of Metallic Vapours. 471 



into sheet and wire when heated at low temperature. 30 per cent, 

 and 40 per cent, with great difficulty only at a temperature little less 

 than melting point, being brittle when cold, but with a grain of great 

 beauty and fineness. 



50 per cent. I have as yet failed to work up into forms other than 

 castings beyond what I can effect by pressure when in a semi-fused 

 condition. 



The general results of my work on this alloy would lead me, there- 

 fore, to make the following recommendations. 



For the manufacture of standard rules to use an alloy of not less 

 than 85 per cent, platinum and 15 per cent, iridium, adopting the 

 tubular form. 



For the standard weights to use an alloy of not less than 80 per 

 per cent, platinum and 20 per cent, iridium, adopting the form now 

 generally made. 



Finally, following the expression of the great French chemist, 

 M. Dumas, I hope by these labours " d'avoir enriche l'ou tillage 

 scientifique d'un alliage doue des proprietes precieuse." 



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

 VI. By G. D. LiVElNG, M.A., Professor of Chemistry, and 

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

 Cambridge. Received March 27, 1879. 



The experiments described in the following communication were 

 made with the electric arc, and in lime crucibles,* or in crucibles of a 

 highly calcareous sandstone, kindly supplied to us by Messrs. Johnson, 

 Matthey, and Co., as described in our fourth communication on this 

 subject ; but for some of them we used, instead of a galvanic battery, 

 a magneto-electric machine producing a much more powerful current 

 and a much longer arc. The experiments with this machine were 

 made, through the kindness of Dr. Tyndall, at the Royal Institution, 

 and we are indebted to Messrs. Siemens both for the working of the 

 machine and for sparing to us the services of a skilled engineer, in 



* In our first paper on this subject, communicated in February, 1878, when re- 

 ferring to the experiments of Lockyer and "Roberts (" Proc. Roy. Soc," xxiii), we 

 mentioned that they employed the combined action of a charcoal furnace and an 

 oxy hydrogen blowpipe, but omitted to mention that they used a lime chamber after the 

 model of Stas. Referring to fig. 1 in our communication of February 12, 1879, where 

 the use of an oxyhydrogen blowpipe in a lime block is represented, we disclaim any 

 novelty in the use of lime ; the difference between our experiments and theirs con- 

 sisting in this, that we use the continuous spectrum from the hot walls of our cruci- 

 ble, instead of an external independent source of light, as a background against which 

 the absorbent action of the vapours is seen, in the same way as we had previously 

 used iron tubes, and now use the electric arc. 



