Action of the Solar Spectrum on Vegetable Colours. 225 



a series of experiments for determining with precision their compa- 

 rative powers of insulation, and of sustaining by induction charges of 

 electricity. The substances to be examined were cast into the form 

 of circular plates and furnished on both their surfaces with circular 

 coatings of tinfoil of a diameter equal to one-half that of the plate, 

 and the electric intensities were measured by electrometers of the 

 same construction as those which he used in his former experiments, 

 and which he has described in his paper* already published in the 

 Philosophical Transactions for 1859. The results are stated in ta- 

 bles ; from the last of which it appears that the inductive capacities 

 of the dialectric bodies tried, that of air being expressed by unity, 

 are proportional to the following numbers : — 



Substances. Relative capacities. 



Air 1 



Rosin 1*77 



Pitch 1-8 



Bees' wax 1*86 



Glass 1-9 



Brimstone 1*93 



Shell-lac 1-95 



The author, in conclusion, offers some observations on the expe- 

 rimental processes employed in his investigation ; and points out 

 several circumstances which require to be attended to in order to 

 ensure success. 



June 16. — The following papers were read, viz. — 

 1. " On the Action of the Rays of the Solar Spectrum on Vegetable 

 Colours." By Sir John F. William Herschel, Bart., K.H., F.R.S. 



The author, having prosecuted the inquiry, the first steps of which 

 he communicated in a paper read to the Royal Society in February 

 1 84-Ot, relating to the effects of the solar spectrum on the colouring 

 matter of the Viola tricolor, and on the resin of guaiacum, re- 

 lates, in the present paper, the results of an extensive series of simi- 

 lar experiments, both on those substances, and also on a great number 

 of vegetable colours, derived from the petals of flowers, and the leaves 

 of various plants. In the case of the destruction of colour of the pre- 

 parations of guaiacum, which takes place by the action of heat, as 

 well as by the less refrangible rays of light, he ascertained that 

 although the non-luminous thermic rays produce an effect, in as far 

 as they communicate heat, they are yet incapable of effecting that 

 peculiar chemical change which other rays, much less copiously en- 

 dowed with heating power, produce in the same experiment. He 

 also found that the discoloration produced by the less refrangible 

 rays is much accelerated by the application of artificial terrestrial 

 heat, whether communicated by conduction or by radiation ; while, 

 on the other hand, it is scarcely or not at all promoted by the purely 

 thermic rays beyond the spectrum, acting under precisely similar cir- 

 cumstances, and in an equal degree of condensation. The author 

 proceeds to describe the photographic effects produced on papers 



[* Noticed in Phil. Mag., Third Series, vol. xv. p. 320— Edit.] 

 [f An abstract of the paper here referred to will be found in Phil. Mag., 

 Third Series, vol. xvi. p. 331. — Edit.] 



Phil. Mag. S. 3. Vol. 21. No. 137. Sept. 1842. Q 



