512 



NATURE 



{April 3, 1879 



the point of the wire is on the prolongation of the axis of 

 tube and disk No. i, and so on. Then, by sliding the 

 candle up till the most luminous part of the flame is 

 level with the point of the wire, it is known that the light 

 will shine full on the disk under experiment. A half 

 cylinder, k, covered with black velvet, protects the 

 candle from draughts. The candle-stand h slides along 

 a straight edge, Im, screwed to the bench, so graduated 

 that by bringing a mark on the sliding stand to one of 

 the divisions, it indicates the number of millimetres 

 separating the surface of the experimental disk from the 

 centre of the candle flame. The experimental powders 

 are laid on one surface of mica or pith disks as a water 

 paint, no cement being used to promote adhesion. Disks 

 of mica or thin metal were punched, while other mate- 

 rials were cut or filed into the shape of disks I7'25 mm. 

 in diameter. 



The exhaustion, which had to be effected after each 

 change of the experimental disks, was carefully brought 

 to the same degree both by actual measurement on a 

 McLeod gauge, and by getting the same repulsion on 

 the standard black disk. In this way all the diff'erent 

 results were fairly comparable one with the other. The 

 presence of aqueous vapour was specially guarded 



against by means of [tubes containing phosphoric an- 

 hydride. 



To show the effect of residual gas inteilding to equalise 

 the amount of repulsion on variously coloured surfaces, 

 I devised an experiment with pith disks, one being lamp- 

 blacked and the other retaining its natural white surface, 

 the standard candle being at the same distance in each 

 case. When the exhaustion is good enough to cause a 

 fair repulsion, the ratio between the amplitude of swing 

 when the black is exposed, and that when the white is 

 exposed, is as 100 : 55"S ; at a little higher exhaustion the 

 ratio is, Black : White : : 100 : 42*5 ; at a still better ex- 

 haustion the ratio is. Black : White : : 100 ; 35. The 

 results of the quantitative examination of the repulsion 

 resulting from radiation when falling on about 100 dif- 

 ferent substances I have arranged in fourteen tables, for 

 details of which I must refer to the Bakerian Lecture for 

 1878, The repulsion is measured, first when no screen is 

 interposed, and secondly, when a cell of water is inserted 

 in the path of the rays. In comparing the two results it 

 must be remembered that the actual amount of repulsion 

 on the standard lampblacked disk, when the water screen 

 is interposed, is only one-twelfth of the amount obtained 

 when no screen is in the way, the distance of candle and 



Fig. 2. 



Other things being equal. In order therefore to compare 

 one with the other the result behind water must be 

 divided by 12. 



Table I. — Results of the Examination of Black Powders 

 Compared with lampblack = 100 these have an aver- 

 age value of 92'2, which becomes 99* i by the interposition 

 of water. 



Table \\.— White Poivders 

 These have an average value of 33 "5, which is reduced 

 to 8'3 behind water. The powerful absorption for the 

 invisible heat rays which white powders exercise is some- 

 what remarkable. Assuming that the ultra red rays from 

 a candle are almost entirely cut off by a water screen, 

 the comparatively strong action (33'5) produced by the 

 naked flame must be mainly due to the absorption of the 

 invisible heat-rays ; and when these are cut off by water, 

 the action is diminished nearly fifty times. With black 

 powders the water only diminishes the action about 

 eleven times. 



Table \\\.—Red Powders 



Amongst these precipitated selenium is noteworthy. 

 To the naked flame its value is 35*8, but when a water 



screen [is interposed, the action becomes 69*5, in com- 

 parison with standard lampblack = 100. Omitting sele- 

 nium, the mean action on red powders without a water 

 screen is 32*2, and with a water screen, 24"9. 



Table IV. — Brown Powders 



Amongst these, peroxide of thallium is remarkable as 

 being repelled under the influence of radiation to a greater 

 extent than any other body hitherto examined, its value 

 being 121 7, in comparison to lampblack = 100. Brown 

 powders behave most like black, the averages of the 

 columns without and with a water screen being 92*7 

 and 94"5. 



Table V. — Yellow Powders 

 Among these, anhydrous tungstic acid resembles scarlet 

 selenium in its anomalous action, the figures being, with- 

 out water, 50-8, and with water, 72*2. The averages of 

 the other yellow powders are 357, and behind water, 

 13-8. 



Table VI. — Green Powders 



These show some discrepancies, which will be referred 

 to farther on. 



