Royal Institution of Great Britain. 131 



the glass. These are called by Mr. Chladni, their discoverer, nodal 

 lines. "When light particles, such as scrapings from the hairs of 

 the bow used, dust, or the powder of lycopodium, happen to be on 

 the plate ; instead of proceeding to the same quiescent lines as the 

 sand, they accumulate at the parts in most violent agitation, form- 

 ing a cloud, and at last settling down into little hemispherical heaps, 

 having a peculiar revolving or involving motion. This determina- 

 tion of light powders has always embarrassed philosophers : and 

 M. Savart has founded a theory of some peculiar modes of vibration 

 upon it. Mr. Faraday's object was to show, that the effect is a 

 very simple arid natural one, consisting of nothing more than cur- 

 rents formed in the air surrounding the plate, which, proceeding 

 from the quiescent to the most agitated parts of the plate, then pass 

 upwards, and in their course carry the light particles with them. 

 Mr. Faraday explained, and demonstrated by numerous experi- 

 ments, how such a current would necessarily result from the manner 

 in which the mechanical forces of the plate are transmitted to the 

 air. He showed that this current could be interrupted by walls of 

 card, when the light particles took new courses. He stated that 

 the heavy particles went to the lines of rest because the air had not 

 force enough to carry them in its course ; but that light particles, 

 being governable by it, were taken in the opposite direction. He 

 confirmed this view by substituting water for air, making the plate 

 vibrate in the former fluid, and showing that the sand was then, 

 carried from the quiescent to the agitated parts, exactly as the lighter 

 particles were in air ; and further, on vibrating plates in vacuo, 

 he found that even the lightest particles went to the lines of 

 rest, because there was no current of air of sufficient force to sweep 

 them in the opposite direction. Want of time prevented Mr. Fara- 

 day from entering upon the explanation of the involving heaps : but 

 this point is fully treated of in his paper read before the Royal 

 Society. He announced that further consideration of the subject 

 induced him to believe he should be able to account, by the same 

 principles, combined with the cohesive power of fluids, for the pecu- 

 liar and hitherto unexplained crispations which occur on water 

 and other fluids lying upon a vibrating plate. 



This being the last evening-meeting of the season, Mr. Faraday, 

 on the part of the committee, took leave of the members, after 

 earnestly exhorting them to use both individual and conjoined exer- 

 tions to aid the prosperity of future seasons.. In the library was 

 placed a beautiful portrait of Sir Humphry Davy, of the full size, 

 copied by W. Pickersgill, jun., from the portrait by Sir Thomas 

 Lawrence. 





