1857.] of Gold (and other Metals) to Light. 395 



In this manner the leaf may be obtained so thin, that I think 

 50 or even 100 might be included in a single progressive undu- 

 lation of light. But the character of the effect on light is not 

 changed, the light transmitted is green, as before ; and though 

 that green tint is due to a condition of the gold induced by 

 pressure, it as yet remains unchanged through all these varieties 

 of thickness and of proportion to the progressive or the lateral 

 undulation. 



Gold-leaf, either fine or common, examined in the microscope, 

 appears as a most irregular thing. It is everywhere closely 

 mottled or striated, according as a part at the middle or the 

 edge of a leaf is selected, minute portions which are close to 

 other parts being four or five times as thick as the latter, if the 

 proportion of light which passes through may be accepted as 

 an indication. Yet this irregular plate does not cause any 

 sensible distortion of an object seen through it, that object being 

 the line of light reflected from a fine wire in the focus of a 

 moderate microscope. Nor perhaps was any distortion due 

 to consecutive convexities and concavities to be expected ; for 

 when the thicker parts of the leaf were examined they seemed 

 to be accumulated plications of the gold, the leaf appearing as 

 a most irregular and crumpled object, with dark veins running 

 across both the thicker and thinner parts, and from one to the 

 other. Yet in the best microscope, and with the highest power, 

 the leaf seemed to be continuous, the occurrence of the smallest 

 sensible hole making that continuity at other parts apparent, 

 and every part possessing its proper green colour. How such 

 a film can act as a plate on polarized light in the manner it 

 does, is one of the queries suggested by the phenomena which 

 requires solution. 



When gold-leaf is laid upon glass and its temperature raised 

 considerably without disturbance, either by the blowpipe or 

 an ordinary Argand gas-burner, it seems to disappear, i. e. 

 the lustre passes away, the light transmitted is abundant and 

 nearly white, and the place appears of a pale brown colour. 

 One would think that much of the metal was dissipated, but all 

 is there, and if the heat has been very high (which is not neces- 

 sary for the best results), the microscope shows it in minute 



of this kind has probably much to do with the formation of the films to be 

 described hereafter. 



