which has just been mentioned will convey the voice distinctly when placed against the skull of the hearer, and will even, according to Dr. Thomas, convey audible speech from the skull of one to that of the other. The efforts to make the audiphone and denta-phone useful as regular instruments of hearing to the deaf have not had satisfactory results. Dr. Thomas acknowledges that the expectations which have been excited on the subject are likely to be disappointed. Those who are able to hear with the aid of the audiphone hear their own voices perfectly without it; while those who are unable to hear their own voices without it can hear nothing with it. Dr. Charles S. Turnbull, of Philadelphia, states in the "Medical and Surgical Reporter" that his experience with these instruments has been as nothing, because the suitable cases were so few and far between. The cases in which they have proved of benefit are cases of acoustic deafness, generally due to middle-ear disease, for which devices of the nature of the ear-trumpet generally afford a more satisfactory remedy than either of the instruments under consideration.
Deterioration of Bookbinding by Illuminating Gas.—Professor William Ripley Nichols publishes an interesting paper on the deterioration of the binding of books in libraries, which is commonly ascribed to the action of sulphuric acid supposed to be generated by burning coal-gas. The agency of sulphuric acid having been disputed by Dr. Wolcott Gibbs and others, Professor Nichols made investigations to determine the question. Having examined a large number of samples of leather in every stage of decay, he found that morocco was but little affected, common sheep binding was attacked, and Russia leather and calf were badly acted upon. An acid taste and an acid reaction were observed that were more marked in proportion as the leather was decayed, and sulphuric acid was found in the extract made from the leather with water, in a similarly increasing proportion. Ammonia was also present, in about such a proportion as in combination with the sulphur would constitute the acid sulphate of ammonia. Samples of fresh leather gave extracts only slightly acid, not enough so to affect the taste, and contained only a minute amount of sulphuric acid in combination. Samples of Russia leather and sheep of good quality yielded from less than a quarter to less than a half of one per cent, of acid, and less than quarter of one per cent, of ammonia. A sample of well-worn but not decayed sheep taken from a Bible more than sixty years old, which had never been exposed to gas, gave 1·42 per cent, of sulphuric acid. Other samples, of very rotten Russia, and of scrapings from a number of books, gave from eight to ten per cent, of sulphuric acid, combined with ammonia. A quantity of rotten leather was carefully extracted with water, and crystals of sulphate of ammonia were obtained from it. It is difficult, in the face of these facts, Professor Nichols urges, to escape the conviction that bindings of Russia, calf, or sheep absorb sulphuric acid when exposed to the products of the combustion of illuminating gas. No other condition to which books are commonly exposed can so well account for the large proportion of acid which was found in the old bindings. It has been objected to this view that sulphurous (not sulphuric) acid is the general product of the combustion of sulphur compounds; but Professor Nichols's analyses of the results of the burning of gas have brought out sulphates with no evidence of the presence of a sulphite. It is admitted to be possible that the disintegration of the leather precedes the absorption of sulphuric acid, and prepares the way for it; and Professor Nichols intends to make experiments for the determination of this question.
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