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I could' only suppose that the formation of a permanent film was the work of time, and some chemical friends were of the same opinion. Accordingly a number of plates were prepared and set aside duly labelled.
Examination at intervals proved that time acted but slowly.    After six months the films seemed more stable, but nothing was obtained comparable with the old iridescent plates.    It is possible that the desired result might eventually be achieved in this way, but the prospect of experimenting under such conditions is not alluring.    Luckily an accidental observation came to my aid.    In order to prevent the precipitation of lime in the observing-dish a few drops of nitric acid were sometimes added to the water, and I fancied that films tested in this acidified water showed an advantage.    A special experiment confirmed the idea.    Two plates, coated similarly with silicate and dried a few hours before, were immersed, one in ordinary tap water, the other in the same water moderately acidified with nitric acid.    After some 24 hours' soaking the first film washed off easily, but the second had much greater fixity.    There was now no difficulty in preparing films capable of showing as good colours as those of the old plates.    The best procedure seems to be to dry off the plates before a fire after coating with recently-filtered silicate solution.    In order to obtain the most suitable thickness, it is necessary to accommodate the rapidity of drying to the strength of the solution.    If heat is not employed the strength of the above given solution may be doubled.     When dry the plates may be immersed for some hours in (much) diluted nitric acid.    They are then fit for optical examination, but are best not rubbed at this stage.    If the colours are suitable the plates may now be washed and allowed to dry.    The full development of the colour effects requires that the back of the plates be treated.    In rny experience grinding gives the best results when the lighting is favourable, but an opaque varnish may also be used with good effect.    The comparative failure of such a treatment of the old plates was due to the existence of films upon both sides.    A sufficiently opaque glass, e.g. stained with cobalt or copper, may also be employed.    After the films have stood some time subsequently to the treatment with acid, they may be rubbed vigorously with a cloth even while wet; but one or two, which probably had been rubbed prematurely, showed scratches.
The surfaces of the new films are not quite as glassy as the best of the old ones, nor so inconspicuous in the air, but there is, I suppose, no doubt that they are all composed of silica. But I am puzzled to understand how the old plates were manipulated. The films cover both sides without interruption, and are thinner at all the four corners than in the interior.
The extraordinary development of the colours in water as compared with what can be seen in air led me to examine in the same way other thin films deposited on glass. A thin coat of albumen (without silicate) is inconspicuous