. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. in their object. Now, as to the value of this set clement it has been al-ready stated that there are some respects in which it mayprove a useful substitute for a strong bath of pure nitrate tion, which probably can then less freely permeate the sur-face of the albumen; hence the tendency is to retain thesensitive material more completely on the surface of thepaper. It is probably in this way that the nitrate of pot-ash acts when added to the nitrate bath, as there can beno question about the
. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. in their object. Now, as to the value of this set clement it has been al-ready stated that there are some respects in which it mayprove a useful substitute for a strong bath of pure nitrate tion, which probably can then less freely permeate the sur-face of the albumen; hence the tendency is to retain thesensitive material more completely on the surface of thepaper. It is probably in this way that the nitrate of pot-ash acts when added to the nitrate bath, as there can beno question about the advantage resulting from its addi-tion to very weak baths; and the new salt is, therefore, aconvenient form in which to use the material for a com-pound bath, and yet one containing a minimum of is another advantage arising from the employmentof a neutral salt as an addition to the printing bath, andthat is the tendency which these bodies possess of pre-venting the albumen, or silver compound of albumen, fromentering the solution. Even apart from the preceding considerations, it is an. Fig. 32. of silver. Its chief advan-tages probably depends onthe improved tone which theprints take when the doublesalt is used to sensitize thepaper. The difference inprice, as the above analysisproves, is only an apparentand not a real one; but thechief point is that, in employ-ing a bath of forty grains ofthe set clement, it is possibleto get results in printingsimilar to those afforded bya bath of nitrate of silver ofnearly eighty grains instrength, although the actualamount of silver salt presentforms but little more thanone-third of the whole. The advantage appears toarise chiefly from the in-creased density of the solu- MEZZOTINT PHOTOGRAPHS. 489 interesting fact that the presence of a salt of magnesia isfound to exercise such a marked influence over the tonewhich the prints take on immersion in the gold bath, andadds an additional fact to the long list which we alreadyhave in
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