the most interesting places, one can hardly lay it away in a portfolio without making some attempt to remove or reduce the strange tints which appear on it. This is the part of my present work most difficult to discuss, while being the most useful. My simple notions of chemistry are not always sufficient and perhaps, some day, some chemist especially trained in analysis and decomposition may, with advantage, rewrite this portion of my work. I will at least record, however, a large number of satisfactory results which I have obtained and even repeated on fragments of proofs on unsized paper, this last being the most unfavorable of all conditions.
The first difficulty comes when the nature of the spot is not easily recognized. This yellow spot which resists both washing and bleaching, may perhaps be formed by some greasy body or by some metallic oxide, and one must proceed carefully on any hypothesis which may be formed. In such cases, where experiments must be tried, it is necessary to know some chem