810 Mr. Faraday [June 13, 



theory the principles of the process justify the expectations, and in 

 practice nothing as yet lias occurred which is counter to them. 



With regard to the second part of the evening's discourse, the 

 speaker said he had been led by certain considerations to seek 

 experimentally for some effect on the rays of light, by bodies 

 which when in small quantities had strong peculiar action upon it, 

 and which also could be divided into plates and particles so thin 

 and minute as to come far within the dimensions of an undula- 

 tion of light, whilst they still retained more or less of the power 

 they had in mass ; and though he had as yet obtained but little new 

 information, he considered it his duty, in some degree, to report 

 progress to the Members of the Royal Institution. The vibrations 

 of light are, for the violet ray 59,570 in an inch, and for the red 

 ray 37,640 in an inch ; it is the lateral portion of the vibration of 

 the ether* which is by hypothesis supposed to affect the eye, but 

 the relation of number remains the same. Now a leaf of gold as 

 supplied by the mechanician is only 2To!7roiT ^^ ^^ moh. in thickness, 

 so that 7^ of these leaves might be placed in the space occupied by 

 a single undulation of the red ray, and 5 in the space occupied by 

 a violet undulation. Gold of this thickness and in this state is 

 transparent, transmitting green light, whilst yellow light is reflected ; 

 there is every reason to believe also that some is absorbed, as hap- 

 pens with all ordinary bodies. When gold leaf is laid upon a layer 

 of water on glass, the water may easily be removed, and solutions 

 be substituted for it ; in this way a solution of chlorine, or of cya- 

 nide of potassium, may be employed to thin the film of gold ; and 

 as the latter dissolves the other metals present in the gold, (silver, 

 for instance, which chlorine leaves as a chloride,) it gives a pure 

 result ; and by washing away the cyanide, and draining and drying 

 the last remains of water, the film is left attached to the glass : it 

 may be experimented with, though in a state of extreme tenuity. 

 Examined either by the electric lamp, or the solar spectrum, or the 

 microscope, this film was apparently continuous in many parts where 

 its thickness could not be a tenth or twentieth part of the original 

 gold leaf. In these parts gold appeared as a very transparent thing, 

 reflecting yellow light and transmitting green and other rays ; it 

 was so thin that it probably did not occupy more than a hundredth 

 part of a vibration of light, and yet there was no peculiar effect 

 produced. The rays of the spectrum were in succession sent through 

 it ; a part of all of them was either stopped or turned back, but 

 that which passed through was unchanged in its character, whether 



* Analogous transverse vibration may easily be obtained on the surface of 

 water or other fluids, by the process described in the Philosophical Transac- 

 tions for 1831, p. 336, &c. 



