1 42 Dr. J. A. Fleming on Molecular Shadows 



rated everywhere. In this case the tendency will always be 

 to increase and go on increasing the resistance, and, therefore, 

 the temperature at the point of highest resistance. Suppose 

 this occurs on the clamp or on the leading-in wire, then ex- 

 perience shows that the metal is volatilized and deposited as a 

 film on the glass. This metallic deposit is not uniform ; it is 

 thickest nearest the base of the lamp and gradually thins away 

 up to the crown, and at a certain height is thin enough to 

 transmit light. It is not very uncommon to obtain Edison 

 lamps with this copper deposit. The colour of transparent 

 copper is a fine sage green inclining to blue ; compared w T ith 

 the colour of gold leaf seen by transmitted light, it very 

 closely resembles it. On one occasion the w r riter obtained a 

 lamp with a silvery metallic film deposited over it on the 

 inside of the glass. From the outside it had a mirror-like 

 lustre; on breaking the lamp this film was seen to be brownish 

 and not brushed off by the finger, but it could be removed by 

 scratching. It was not removed by holding in the oxidizing 

 flame of a Bunsen burner: therefore it was not carbon. It 

 was not removed by nitric acid; but on boiling a fragment of 

 the glass, covered with this metallic deposit, in nitro-hydro- 

 chloric acid the film disappeared. It was therefore probably 

 platinum. The film was transparent, permitting objects to be 

 seen through it, and transmitted brownish light. 



Now, under certain circumstances, a line of no deposit is 

 formed on the surface in the plane of the filament, which is, 

 as it were, the shadow of one side of the loop This indicates 

 that the process of molecular scattering, which is going on at 

 some spot on one or other clamp, is not a mere evaporation or 

 volatilization of the metal, but a projection of molecules in 

 straight lines in every direction. The trajectory of the mole- 

 cules will be interfered with in some directions by the carbon 

 filament; and hence result lines and places of no deposit which 

 are molecular shadows of the loop. On every other part of 

 the glass the molecules will inpinge and adhere, forming a 

 metallic coating. From the facts that the free paths of the 

 molecules differ in length, and that the clamp is much nearer 

 to the neck of the lamp than to the crown, it follows that a 

 much larger proportion of the scattered molecules strike the 

 glass near the neck, and the thickness of the deposit is there- 

 fore a measure of the proportion of molecules which have a 

 free path, equal to the distance of that part of the envelope 

 from the scattering point. Curiously enough the line of no 

 deposit, or shadow of the loop, is not always seen in copper- 

 deposited lamps. This may be because the scattering is 

 going on from both clamps, and therefore the shadow on one 



