532 Prof. Faraday on the Experimental Relations 
be added to the warm jelly, and the latter be then mixed gra- 
dually and with agitation with the gold solution, a ruby jelly is 
generally produced. In such ruby jelly the reduced particles of 
gold preserve their state and relative place, and the tint does not 
pass to blue, even though a considerable proportion of salt be 
present. Such jelly will remain in the air for weeks before it 
decays, and has every character, in colour and appearance, of 
gold ruby glass. It is hardly possible to examine the series of 
ruby glass, ruby membrane, ruby jelly cold and gelatinous, ruby 
jelly warm and fluid, and the ruby fluids, to consider their pro- 
duction, and then to conclude that the cause of their common 
ruby colour is not the same in all. 
When the warm ruby jelly is poured into a capsule or on toa 
plate, allowed to gelatinize and then left in the air, it gradually 
becomes dry. When dry, some of these jellies remain ruby; 
others will probably be of an amethystine violet colour, or perhaps 
almost blue. When one of the latter is moistened with water, 
and has absorbed that fluid, it becomes gelatinous, and whilst in 
that state resumes its first ruby colour; but on being suffered 
to dry again, it returns to its amethystine or blue colour. This 
change will occur for any number of times, as often as the jelly 
is wetted and dried. Here the gold remains in the same me- 
tallic state through this great change of colour, the association or 
the absence of water being the cause: and the effect strengthens 
in my mind the thought before expressed, that in the ruby 
fluids the deposited particles are frequently associates of water 
and gold. It isa striking case of the joint effect of the media 
and the gold in their action on the rays of light, and the most 
striking case amongst those where the medium may be changed 
to and fro. 
When a ruby jelly is prepared with salt, and being warm is 
poured out in thin layers on to glass or porcelain, it first gela- 
tinizes and then dries up ; in which case the salt is excluded and 
crystallizes. When the dry jelly is put into cold water, the salt 
dissolves and can beremoved. ‘The jelly then swells to a certain 
amount, after which it can be left soaking in water for a week or 
longer, until everything soluble is separated. No change takes 
place in the ruby tint, no goldisremoved. When the last water 
is poured off and the remaining jelly warmed, it melts, forming 
a fine ruby fluid, which can either be dissolved in more water, or 
regelatinized, or be dried and preserved for any length of time. 
It is perfectly neutral ; gives no signs of dissolved gold by any 
of the tests of the metal; is not changed by sulphuretted hy- 
drogen, gallic acid, pyrogallic acid, dilute caustic alkalies, or 
carbonated alkalies or lime-water ; or by dilute sulphuric, hydro- 
chloric or nitric acids, the actions being continued for fourteen 
