It is well known that a large number of vegetable substances are employed as tanning agents. Our researches have been directed to leather tanned by means of the most important of these agents. Many questions present themselves in the course of such an examination. Among others, that most important one, from a practical point of view, of the weight the tanning agent gives to the hide, that is to say, the result in leather of weight given to the raw material. The degree of tannage is also to be considered; the length of time during which the tanning agent is to be left with the hide; in short, the influence upon the leather of the substances used in its production. That is why we have made the completest possible analysis of different leathers. Besides ordinary oak bark there are used at present very different substances, such as laurel, chestnut, hemlock, quebracho and pine bark, sumac, etc. Water is an element that exists in all hides, and it is necessary to take it into consideration in the analysis. It is present in perceptible quantity even in dry hides. This water cannot be entirely eradicated without injuring the leather, which will lose in suppleness and appearance. Water should then be considered as one of the elements of leather, but it must be understood that if it exceeds certain limits, say 12 to 14 per cent., it becomes useless and even injurious. Moreover, if there is any excess over the normal quantity, it becomes deceptive and dishonest, as in such a case one sells for hides that which is nothing but water. Supposing that a hide, instead of only 14 per cent., contained 18 per cent. of water, it is evident that in buying 100 pounds of such a hide one would pay for four pounds of water at the rate for which he purchased the hide. There are, also, some matters soluble in air, which are formed to a large extent from fat arising as much from the hide as from tanning substances. The air dissolves at the same time a certain amount of organic acid and resinous products which the hide has absorbed. After treating with air, alcohol is used, which dissolves principally the coloring matters, tannin which has not become assimilated, bodies analogous to resin, and some extractive substances. That which remains after these methods have been pursued ought to be regarded as the hide proper, that is to say, as the animal tissue saturated with tannic acid. In this remainder one is able to estimate with close precision that which belongs to the hide. The hide being an elementary tissue of unchangeable form, it is easy, in determining the elementary portion, to find the amount of real hide remaining in the product. With these elements one can arrive at a solution of some of the questions we are discussing. We give below, according to this method, a table showing the composition of the different leathers exhibited at the Paris Exposition of 1878. They are the results of careful research, and we have based our work upon them: