April Z, 1879] 



NATURE 



513 



Table WU.— Blue Powders 

 These show a much stronger proportionate action be- 

 hind a water screen than with no screen, the averages 

 being 55 "8 and 6S"2. 



Table VIII. — Dyes and Colouring Mailers of Organic 

 Origin 



Among these may be noticed saffranin, and a product 

 of the decomposition of chlorophyll, which show an in- 

 creased ratio or action when the heat-rays are cut off by 

 water. Leaving out these, the mean actions of the other 

 substances are, with no screen, 44*5, with a water-screen 

 interposed, i%'\. 



Table IX. — Metals prepared in different Ways ajid 

 coated with Lampblack, Mica, Qr'c. 



Curious results are shown with iron and with gold, the 

 former metal chiefly absorbing the invisible heat rays, 

 whilst the latter metal is principally acted on by the 

 luminous rays. 



Tables X. and Xa. — Various Silver Salts 



The chloride, bromide, and iodide of silver in their 

 different states were exposed to the standard candle after 

 being submitted to the action of magnesium light, sun- 

 light, and daylight. The results show how readily a 

 change in the state of the surface is detected by an 

 increased amount of repulsion under the influence of 

 radiation. 



Table XI. — Selenium— Crystalline and Vitreous 



The former is in the state most sensitive to light action* 

 With the crystalline disk results have been obtained 

 which seem to show that the impact of light on its sur- 

 face produces a superficial disturbance there and in the 

 adjacent gaseous molecules, which takes some time to 

 subside. This is connected with the change in electric 

 conducting power of crystalline selenium — a change which, 

 when the element is transferred from light to darkness, 

 also takes some time to subside. 



Table XII. — Miscellaiuous Substances — Pith, Mica, 

 Charcoal, Glass 



The complicated nature of these actions was well shown 

 in the results I obtained with three pith disks, the first 

 being plain white, the second lampblacked on the front, 

 and the third lamblacked on the back. The first was 

 repelled with a power of 177, the second, which was the 

 standard, with a power of 100, whilst the third was not 

 moved at all. The repulsion exerted on the white surface 

 must have been the same in each case, but the pressure 

 behind the pith caused a radiation of heat from the back 

 surface, which produced molecular pressure just sufficient 

 to neutralise the pressure in front. 



To show that physical condition has more efifect in 

 causing repulsion than chemical composition, I experi- 

 mented with various kinds of charcoal. I found that the 

 repulsion suffered by cocoa-nut shell charcoal is much 

 less than that of white pith, being only 11 -6 against 177, 

 At the same time a radiometer made of cocoa-nut shell 

 charcoal, lampblacked on one side, was only moderately 

 sensitive, instead of being superior to one made of pith 

 lampblacked on one side. The low figure shown by the 

 charcoal was caused by its density enabling it to conduct ! 

 heat from one surface to the other. Molecular pressure j 

 is therefore generated on both the back and front surfaces, ! 

 and the figure I obtained is simply the difference between i 

 the two opposing actions. I 



I used other screens, besides water, to filter the radia- I 

 tion of the candle before it fell on the disk. I, however, j 

 preferred water. It is almost perfectly opaque to the in- 

 visible heat rays, and therefore its employment allows 

 easy discrimination between actions due to heat and to : 

 heat and light combined ; secondly, it is colourless, and ' 



having no selective action on any visible ray of light, it 

 can be used in conjunction with any coloured powder 

 without complicating the results. Alum acts in a similar 

 manner to water ; coloured solutions act as water with a 

 super-added action due to their colour. Very thick 

 plates of glass have less action on the invisible heat rays 

 than a thin layer of water. Sulphate of copper, in a solution 

 so weak as to appear only slightly green, has a very 

 strong action when artificial light is used, as it cuts off 

 the lowest visible red rays as well as the ultra red. 



I found that the substances I had experimented on 

 might be divided into two classes. 



1. Negative, those in which the repulsion behind water 

 is greater in proportion to the standard than when no 

 screen is present. 



2. Positive, those iri which the repulsion in proportion 

 to the standard is less behind water than when no screen 

 is present. 



Amongst Class i may be mentioned copper tungs- 

 tate, saffranin, scarlet selenium, and copper oxalate ; 

 these are more affected by light than by invisible heat. 

 Amongst Class 2 I may mention pale green chromic 

 oxide, persulphocyanogen, hydrated zinc oxide, bariimi 

 sulphate, and calcium carbonate; these substances are 

 more acted on by the ultra-red rays than by the luminous 

 rays. To render these differences of action more com- 

 parable, I divided the averages obtained by the water 



C^ntivxetres. 

 Fig. 3. 



screen by twelve. Uniting the two classes together, the 

 figures then became as follows : — 



Water screen 



An examination of this table shows that the results can be 

 proved by balancing one powder against another in a radio- 

 meter. A bulb was therefore blown on the end of a wide 

 tube, as shown at Fig. 3. The top of the bulb was opened 

 and turned over to form a lip ; this was ground smooth and 

 polished, so as to be readily closed by cementing on it a 

 piece of plate glass. A glass stem supports a fine needle 

 in the centre of the bulb, and on this rests a glass cap, to 

 which is attached four radial arms of aluminium. To 

 these arms disks of mica or pith can be fastened so as to 

 form the movable fly of a radiometer. The disks can be 

 changed by unceraenting the glass top, and lilting the fly 



