1878.] 



resulting from Radiation. 



33 



fcnngstate, saffranin, scarlet selenium, and copper oxalate ; these are 

 more affected by light than by invisible heat. Amongst class 2 

 may be mentioned pale green chromic oxide, persulphocyanogen, 

 hydrated zinc oxide, barium sulphate, and calcium carbonate ; these 

 substances are more acted on by the ultra red rays than by the 

 luminous rays. 



From the results given in these tables, the author has been led to 

 the construction of several radiometers which afford ocular proof of 

 the general correctness of the indications given by the water screen. 

 Thus, a radiometer made of pith disks coated on alternate sides with 

 chromic oxide and precipitated selenium, moves in one direction to 

 the naked flame of a candle and in the other direction when a water 

 screen is interposed. A similar radiometer coated with saffranin 

 and hydrated zinc oxide does not move at all when exposed to the 

 naked flame, but revolves when a water screen is interposed. A 

 radiometer coated with thallic oxide and Magnus's green platinum 

 salt moves strongly when no screen is interposed, but is stopped by a 

 water screen. Many other instruments having similar behaviour to 

 the above are described, and their actions discussed and explained. 

 The following experiment is given ; a pith radiometer, coated with pre- 

 cipitated selenium and chromic oxide, is exposed to the radiation from 

 a colourless gas flame from a Bunsen burner, coloured intensely green 

 by thallium. To the eye, by this light, the chromic oxide looks nearly 

 white and the scarlet selenium black. The rotation due to the 

 superior repulsion of the chromic oxide is, however, apparently as 

 strong as when the non-luminous flame is used. This is a proof that 

 the train of reasoning the author has employed on former occasions is 

 correct, viz., that certain substances have an absorptive action on rays 

 of dark heat opposite to what they have on light, and that an optically 

 white body may be thermically black, and vice versa. Here, for 

 instance, chromic oxide is optically green and thermically black, 

 whilst scarlet selenium is thermically white. 



The author next examines the amount of repulsion produced when 

 polarised light is allowed to fall on a plate of tourmaline suspended in 

 vacuo in a torsion balance. It was considered possible that the repul- 

 sion might vary according to the plane of polarisation of the incident 

 ray. Three separate sets of experiments are given in detail, and the 

 results discussed ; the final decision being that the special action 

 originally thought possible does not exist in a degree appreciable with 

 the present experimental means. 



The next portion of the paper is devoted to an examination of the 

 action of radiation on radiometers, the vanes of which consist of 

 metallic plates, either bright on both sides or blacked on one side, and 

 turned up at the corners in different degrees. When flat plates are 

 taken, blacked on alternate sides, the rotation is normal or positive, 



vol. xxvii. D 



