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222 THE ADVANCE OF PHOTOGRAPHY
time subchloride of mercury (mercurous chloride), which contains less chlorine than the chloride of mercury, is precipitated. This body is also white, and therefore invisible on the white paper. Now there are several substances which colour this white subchloride of mercury black. Among these are ammonia and hypo-sulphite of soda. If, therefore, the invisible picture is moistened with one of these substances, it is coloured black and becomes visible. In the magic photographs formerly sold there was hypo-sulphite of sodium in the blotting-paper ; this was dissolved on moistening the paper, the solution penetrated to the picture and made it visible.
Quite a different kind of magic photograph was offered for sale some years later — the magic cigar-holders. These •contained a small sheet of paper between the cigar and the mouthpiece, which was exposed to the action of the cigar smoke ; with continued smoking an image became visible on the sheet of paper, which contained a magic photograph of the kind described above. The image was brought out by the vapour of ammonia which is contained in the smoke, which has also the property of colouring the magic photographs black.
These magic photographs were introduced at Berlin by Grime, but their principle was known before, as J. Herschel had produced similar ones in 1840.
Scamoni s Heliographic Process. — Scamoni, a heliographer of the Russian Imperial State Paper Office, observed that a photographic negative does not form a plane surface, but appears in relief, the transparent places — shadows — being deeper than the opaque. But the difference is very slight. Scamoni tried to increase it by treating the freshly taken and developed picture with pyrogallic acid and solution of silver. In this manner fresh silver was precipitated on the picture, which has the property of attracting and retaining silver separated chemically. The relief was considerably