June 26, 1890] 



NATURE 



199 



sidered to be projecting off molecules of carbon in all 

 ■directions, partly in virtue of purely thermal actions, but 

 probably also in consequence of certain electrical effects 

 to be presently discussed. This scattering of the mate- 

 rial of the carbon conductor takes place with disadvant- 

 ageous rapidity from an industrial point of view at and 

 beyond a certain temperature/ but it exists as well at 

 much lower temperatures than that which is found to 

 determine the practical limit of durability. A curious 

 appearance is found in many incandescent lamps which 

 have been " over-run," which shows us that this projec- 

 tion of carbon molecules from the hot conductor is not, 

 perhaps, best described by calling it a vaporization of its 

 substance, but that the surface molecules are shot off in 

 straight lines, and that they reach the glass envelope 

 without being hindered to any great extent by the mole- 

 <:ules of the residual air. 



If an electric current is passed through an otherwise 

 uniform carbon conductor, which possesses at any one 

 place a specific resistance higher than that of the remain- 

 ing portion, the current, in accordance with a well-known 

 law, there develops a higher temperature, and the mole- 

 cular scattering at that spot may in consequence be 

 greatly exaggerated. It may be that the detrition of the 

 conductor at that locality will be so great as to cut it 

 through after a very short time. When the carbon has 

 the form of a simple horseshoe loop, and when this mole- 



'-tt 



Fig. I.— Glow-lamp, having the gLiss bulb blackened by deposit of carbon, 

 showing the molecular scattering which has taken place from the point 

 a on the filament, and the shadow or line of no deposit produced at b. 



cular scattering takes place from some point in the middle 

 of one branch, the molecular projection makes itself evi- 

 dent by producing a " molecular shadow " of the other leg 

 upon the interior of the glass. I will project upon the 

 screen an image of the carbon horse-shoe loop taken 

 from an old glow-lamp, and you will be able to see that 

 the filament has been cut through at one place. At that 

 position some minute congenital defect caused the carbon 

 to have a higher resistance, the temperature at that point 

 when it was in use became excessive, and an intensified 

 molecular scattering took place from that locality. On 

 examining the glass bulb from which it was taken, we 

 find that the glass has been everywhere darkened by a 

 deposit of the scattered carbon except along one narrow 

 line (see Fig. i), and that line is in the plane of the car- 

 bon loop and on the side opposite to the point of rupture 

 of the filament.^ 



• When_ the rate of expenditure of energy in the carbon conductor is 

 raised until it reaches a value of absut 500 watts, or 360 foot-pounds per 

 second per square inch of radiative surface, a limit of useful temperature 

 has been reached for economical working, under the usual present conditions 

 of steam-engine-driven dynamos and modern glow-lamps. 



' The writer desires to express his indebtedness to the editor of the 

 Electrician for the loan of the blocks illustrating this abstract. 



NO. 1078, VOL. 42] 



I may illustrate to you, by a very simple experiment, 

 the way in which that " shadow " has been formed. Here 

 is a n-shaped rod : this shall represent the carbon con- 

 ductor in the lamp ; this sheet of cardboard placed behind 

 it, the side of the glass receiver. I have affixed a little 

 spray-producer to one side of the loop, and from that 

 point blow out a spray of inky water. Consider the ink 

 spray to represent the carbon atoms shot off from the 

 overheated spot. We see that the cardboard is bespattered 

 on all points except along oneline where it is sheltered by 

 the opposite side of the loop. We have thus produced a 

 " spray shadow " on the board (Fig. 2). The existence 

 of these molecular shadows in incandescent lamps leads 

 us therefore to recognize that the carbon atoms must be 

 shot off in straight lines, or else obviously no such sharp 

 shadow could thus be formed. This phenomenon confirms 

 in a very beautiful manner the deductions of the kinetic 

 theory of gases. I may remind you that at the ordinary 

 temperature and pressure the mean free path of a mole- 

 cule of air is deduced to be about four one-millionths of 

 an inch. This is the average distance which such a 

 gaseous molecule moves over before meeting with a 

 collision against a neighbour which changes the direction 

 of its path. Let the air be rarefied, as in these bulbs, to 

 something like a millionth of the ordinary atmospheric 

 pressure, and the mean free path is increased to several 

 inches. The space within the bulb — though from one 

 point of view densely populated with molecules of residual 

 air — is yet, as a fact, in such a condition of rarefaction 

 that a carbon molecule projected from the conductor can 

 move over a distance of three or four inches on an average 



Fig. 2. — " Spray shadow " of a rod thrown on cardboard screen to illustrate 

 formation of "molecular shadow " in glow-lamps. 



without meeting with interference by collision with another 

 molecule, and the facts revealed to us by these shadows 

 show that this must be the case. I have also at hand some 

 Edison lamps in which these " molecular shadows " are 

 finely shown, but in these cases the deposit on the in- 

 terior of the bulb is not carbon but copper, because the 

 molecular scattering has here taken place by excessive 

 temperature developed at the copper clamps by which the 

 carbon filament is attached to the platinum wires. The 

 theory, however, is the same. The deposit of copper 

 shows a fine green colour by transmitted light in the 

 thinner portions. One curious lamp also before me had 

 by an accident an aluminium plate volatilized within the 

 bulb. The glass receiver has in consequence been 

 covered with a mirror-like deposit of aluminium, which 

 on the thinner portions shows a fine blue colour by trans- 

 mitted light, and a silvery lustre by reflected light. This 

 lamp also shows a fine " molecular shadow." 



These facts prepare us to accept the view that when a 

 glow-lamp is in operation the highly rarefied residual air 

 in the interior of the bulb is being traversed in all 

 directions by multitudinous carbon atoms projected off 

 from the incandescent carbon conductor. I now wish to 

 pass in review before you some facts which indicate that 

 these carbon atoms carry with them electric charges, and 

 that they are charged, if at all, with negative electricity. 



