Aprils, 1876] 



NATURE 



451 



100, the following are the mechanical values of the dif- 

 ferent colours of the spectrum : — 



Ultra-red 



Extreme red 



Red ... 



Orange 



Yellow 



Green 



Blue ... 



Indigo 



Violet 



Ultra-violet 



100 

 85 

 73 

 66 



57 



41 



22 



%\ 



6 



5 



A comparison of these figures with those usually given 

 in text-books to represent the distribution of heat in the 

 spectrum is a sufficient proof that the mechanical action 

 of radiation is as much a function of the luminous rays 

 as it is of the dark heat-rays. 



The author discusses the question, " Is the effect due 

 to heat or to light ?" There is no real difference between 

 heat and light ; all we can take account of is difference of 

 •wave-length ; and a ray of a definite refrangibility cannot 

 be split up into two rays, one being heat and one light. 

 Take, for instance, a ray of definite refrangibility in the 

 red. Falling on a thermometer it shows the action of 

 heat, on a thermopile it produces an electric current, to 

 the eye it appears as light and colour, on a photographic 

 plate it causes chemical action, and on the suspended 

 pith it causes motion. But all these actions are insepa- 

 rable attributes of the ray of that particular wave-length, 

 and are not evidence of separate identities. 



The author enters into some theoretical explanations of 

 the action of the different parts of the spectrum, but these 

 cannot well be given in abstract. 



An experiment is described by which sunlight was fil- 

 tered through alum, glass, and water screens, so as to cut 

 off the whole of the ultra- red or dark-heat rays. The ray 

 of light which was thus freed from dark heat was allowed 

 to fall on the pith surface of the torsion-apparatus, when 

 it produced a deflection of 105°. On interposing a solu- 

 tion of iodine in disulphide of carbon the deflection fell 

 to 2°, showing that the previous action was almost entirely 

 due to /ijf/it. With a candle tried under the same circum- 

 stances, the light filtered from dark heat produced a 

 deflection of 37°, which was reduced to 5° by interposing 

 the opaque solution of iodine. 



In order to obtain comparative results among discs*of 

 pith coated with lampblack and with other substances, a 

 torsion-apparatus was constructed in which two or more 

 discs could be exposed one after the other to a standard 

 light. One disc always being lampblacked pith, the other 

 discs could be changed so as to get comparisons of action. 

 If the action of radiation from a candle on the lamp- 

 blacked disc be taken as 100, the following are the pro- 

 portions obtained : — 



On Lampblacked pith ... 

 Iodide of palladium ... 

 Precipitated silver 

 Amorphous phosphorus 

 Svdphate of baryta ... 

 MilJc of sulphur 

 Red oxide of iron ... 

 Scarlet iodide of mercury and copper 

 Lampblacked silver ... 



White pith 



Carbonate of lead 



Rock-salt 



Glass 



100 



87-3 



56 



40 



37 

 31 

 28 

 22 

 iS 

 18 



13 

 6-5 



6-5 



In consequence of some experiments tried by Professors 

 Tait and Dew^r, and published in Nature, vol. xii. p. 

 217, the author fitted up a verj- sensitive apparatus for the 

 purpose of carefully examining the action of radiation on 

 alum, rock-salt, and glass. The source of radiation was 

 a candle. Perfectly transparent and highly polished 

 plates of the same size were used, and the deflection was 



made evident by an index ray of light. Taking the action 

 on the alum at 100, that on the rock-salt in five successive 

 exf>eriments was 81, 77'3, 71, 62*5, 6o"4. This increasing 

 action on the alum was found to be caused by efflor- 

 escence, which took place rapidly in the vacuum, and 

 rendered the crystal partially opaque. A fresh alum 

 plate being taken, this and the rock salt were coated with 

 lampblack and replaced in the apparatus, the black side 

 away from the source of radiation, so that the radiation 

 would pass through the crj-stal before reaching the lamp- 

 black. The action of radiation was in the proportion of 

 blacked alum 100 to blacked rock-salt 73. 



Rock-salt and glass were next tested against each other 

 in vaaio in a torsion-balance. Professors Dewar and 

 Tait say that rock-salt is inactive when the beam from a 

 candle is thrown on it, while a glass disc is active. The 

 author has failed to corroborate these results ; he found 

 the mean of several concordant observations to be — rock- 

 salt 39, glass 40. 



The Measurement of the Force. — The author de- 

 scribes a torsion-balance in which he is enabled to 

 weigh the force of radiation from a candle, and give 

 it in decimals of a grain. The principle of the in- 

 strument is that of Xv. Ritchie's torsion-balance, de- 

 scribed in the Philosophical Transactions for 1830. The 

 construction is somewhat complicated, and cannot be 

 well described without reference to the diagrams which 

 accompany the original paper. A light beam, having two 

 square inches of pith at one end, is balanced on a very 

 fine fibre of glass stretched horizontally in a tube, one end 

 of the fibre being connected with a torsion-handle passing 

 through the tube, and indicating angular movements on a 

 graduated circle. The beam is cemented to the torsion- 

 fibre, and the whole is enclosed in glass and connected 

 with the mercury-pump and exhausted as perfectly as 

 possible. A weight of o"oi grain is so arranged that it 

 can be placed on the pith or removed from it at pleasure. 

 A ray of light from a lamp reflected from a mirror in the 

 centre of the beam to a millimetre-scale 4 feet off shows 

 the slightest movement. W^hen the reflected ray points 

 to zero, a turn of the torsion-handle in one or the other 

 direction will raise or depress the pith end of the beam, 

 and thus cause the index ray to travel along the scale to 

 the right or to the left. If a small weight is placed on 

 one end so as to depress it, and the torsion-handle is then 

 turned, the tendency of the glass fibre to untwist itself will 

 ultimately balance the downward pressure of the weight, 

 and will again bring the index ray to zero. It was found 

 that when the weight of the j^ of a grain was placed on 

 the pith surface, the torsion-handle had to be turned 

 twenty-seven revolutions and 353°, or 10073° before the 

 beam became horizontal. The downward pressure of the 

 io(J of ^ grain was therefore equivalent to the force of 

 torsion of the glass thread when twisted through 10073°. 



The author next ascertained what was the smallest 

 amount of weight which the balance would indicate. He 

 found that 1° of torsion gave a very decided movement of 

 the index ray of fight, a torsion of 10073° balancing the 

 ^^ of a grain, while 10074° overbalanced it. The balance 

 will therefore turn to the TtnitftfWTnr of ^ grain. 



Divide a grain weight into a million parts, place one of 

 them on the pan of the balance, and the beam will be in- 

 stantly depressed. 



Weighed in this balance the mechanical force of a 

 candle 12 inches off was found to be 0000444 gH'^in ; of 

 a candle 6 inches off 0-001772 grain. At half the distance 

 the weight of radiation should be four times, or c'ooi776 

 grain ; the difference between theory and experiment 

 being only four miUionths of a grain is a sufficient proof 

 that the indications of this instrument, fike those of the 

 apparatus previously described by the author, follow 

 rigidly the law of inverse squares. An examination of 

 the differences between the separate observations and the 

 mean shows that the author's estimate of the sensitiveness 



