TRANSACTIONS OP THE SECTIONS. 17 



aggregate themselves soon become too heavy to be sustained, and so they fall down. 

 The same mode of explanation, when carried a step further, shows the reason of the 

 curious motions commonly observed in the film of wine adhering to the inside 

 of a wine-glass, when the glass, having been partially filled with wine, has been 

 shaken so as to wet the inside above the general level of the surface of the liquid ; 

 for, to explain these motions, it is only necessary further to bring under consideration, 

 that the thin film adhering to the inside of the glass must very quickly become more 

 watery than the rest on account of the evaporation of the alcohol contained in it 

 being more rapid than the evaporation of the water. On this matter, Mr. Thomson 

 exhibited to the Section a very decisive experiment. He showed that in a vial partly 

 filled with wine, no motion of the kind described occcurs as long as the vial is kept 

 corked. On his removing the cork, however, and withdrawing by a tube the air 

 saturated with vapour of the wine, so that it was replaced by fresh air capable of 

 producing evaporation, a liquid film was instantly seen as a horizontal ring creeping 

 up the interior of the vial, with viscid-looking pendent streams descending from it like 

 a fringe from a curtain. He gave another striking illustration by pouring water on 

 a flat silver tray, previously carefully cleaned from any film which could hinder the 

 water from thoroughly wetting the surface. The water was about one- tenth of an 

 inch deep. Then, on a little alcohol being laid down in the middle of the tray, the 

 water immediately rushed away from the middle, leaving a deep hollow there, which 

 laid the tray bare of all liquid, except an exceedingly thin film. These and other 

 experiments, which he made with fine lycopodium powder dusted on the surface of 

 the water, into the middle of which he introduced alcohol gently from a fine tube, 

 were very simple, and can easily be repeated. Certain curious return currents which 

 he showed by means of the powder on the surface, he stated he had not yet been able 

 fully to explain. He referred to very interesting phsenomena previously observed by 

 Mr, Varley, and described in the fiftieth volume of the Transactions of the Society 

 of Arts, and he believed that many or all of these would prove to be explicable 

 according to the principles he had now proposed. 



On the Effects of Mechanical Strain on the Thermo-Electric Qualities of 

 Metals. Bij Professor W. Thomson, M.A., F.R.S. 



Having found by experiment that iron and copper wires, when stretched by forces 

 insufficient to cause any permanent elongation, had their thermo-electric quahties 

 altered, but immediately fell back to their primitive condition in this respect when 

 the stretching forces were removed ; having remarked that these temporary effects 

 were in each case the reverse of the permanent thermo-electric effects previously 

 discovered by Magnus, as resulting from permanent elongation of the wires, by 

 drawing them through holes in a draw-plate ; and thinking it most probable that 

 all these effects depended on mechanical induction of the thermo-electric qualities of 

 a crystal in the metals operated upon ; the author undertook an experimental inves- 

 tigation of the thermo-electric effects of mechanical strains, in which he intended to 

 include longitudinal extension, longitudinal compression, lateral compression, and 

 lateral extension, and in each case to test both the temporary eflects of strains 

 within the elastic limits of the substance, and the residual alterations in thermo- 

 electric quality, manifested after the cessation of the constraining force, when this 

 has been so great as to give the substance a permanent set. The cycle of experi- 

 ments has now been so nearly completed for both the temporary and the permanent 

 strains, as to allow the author to conclude with certainty that the peculiar thermo- 

 electric qualities induced in each case are those of a crystal. Thus, he finds that 

 iron bars, hardened by longitudinal compression, have the reverse thermo-electric 

 property to that discovered by Magnus in iron wires hardened by drawing ; and 

 that iron wire, under lateral compression, manifests the same thermo-electric pro- 

 perty as the author had discovered in iron wire while under a longitudinal stretch- 

 ing force. The apparatus by which these results were obtained was exhibited to the 

 Section, and the mode of experimenting fully described. As regards iron, the 

 general conclusion is, that its thermo-electric quality, when under pressure in one 

 direction, deviates from that of the unstrained metal, towards bismuth for currents 

 in the direction of the strain, and towards antimony for currents perpendicular to 



1855. 2 



