for as both lights—the direct light of the sun, as well as the derived light of the sky—kindle Bologna phosphorus, so both act on coloured surfaces. Whether the light attacks the colour allied to it, and, as it were, kindles and consumes it, thus reducing the definite quality to a general state, or whether some other operation, unknown to us, takes place, it is clear that light exercises a great power on coloured surfaces, and bleaches them more or less. Here, however, the different colours exhibit a different degree of durability; yellow, especially if prepared from certain materials, is, in this case, the first to fly.
597.
Not only light, but air, and especially water, act strongly in destroying colour. It has been even asserted that thread, well soaked and spread on the grass at night, bleaches better than that which is exposed, after soaking, to the sun-light. Thus, in this case, water proves to be a solving and conducting agent, removing the accidental quality, and restoring the substance to a general or colourless state.
598.
The extraction of colour is also effected by re-agents. Spirits of wine has a peculiar tendency to attract the juice which tinges plants, and becomes coloured with it often in a very permanent manner. Sulphuric acid is very efficient in removing colour, especially from wool and silk, and every one is acquainted with the use of sulphur vapours in bleaching.
599.
The strongest acids have been recommended more recently as more expeditious agents in bleaching.
600.
The alkaline re-agents produce the same