small proportion of color, a hard and intense negative may be made to yield soft prints. As a normal proportion, however, for good negatives, two per cent, of carbon is sufficient. Of course, the proportion of pigment required is different with different pigments, and depends upon the opacity and colorific power of the color employed. Mr. Swan prefers the use of insoluble pigments, as when the tissue is prepared with soluble colors the prints are apt to lose a little of depth and force, if they are subjected to prolonged washing.
THE GELATINE.
Gelatine, as found in commerce, is a very variable substance, and is often impure. It is difficult to give a rule for its selection; but, speaking generally, the gelatine sold for culinary purposes answers well for carbon printing. Common glue is not suitable, and the best samples of gelatine used in cookery are unnecessarily expensive. Different gelatines vary considerably, both in the proportions required, and in the results they produce. Some samples of commercial gelatine, and some of glue, show a tendency to dissolve in cold water. These are unsuitable for the process. Impurities, such as alum or acid, are highly objectionable.
A HINT ON THE PREPARATION OF SOLUTION OF INDIA-RUBBER IN BENZOLE.
A scientific friend has directed our attention to a curious circumstance which he has observed in preparing a solution of India-rubber in benzole; and, as many of our readers have met with difficulties in the direction which we shall now mention, we throw out the hint here. Our friend had great difficulty in preparing a
