Gas can also be eliminated through the skin—carbonic acid—though in infinitely smaller quantity than by the lungs, for while the latter eliminate from 800 to 1200 grammes of carbonic acid, it has been shown by the experiments of Aubert[232] that a human being eliminates through the skin a maximum of 6.3 grammes and a minimum of 2.3 grammes in twenty-four hours, which is certainly a very small quantity. Besides carbonic acid the skin probably also eliminates other organic combinations in the form of gas, though it would be very difficult to analyze them by exact experiments. Pettenkoffer was able to demonstrate from experiments, that, if many persons are in a confined place, it is not carbonic acid alone that causes the very disagreeable sensation of the foul air, but that it is a consequence of the accumulation of harmful products of perspiration through the skin, the particular nature of which has not been determined as yet. Footnote 232: Aubert: Pflüger Archiv, vol. vi, p. 539, 1872. On this account it is permissible to speak of a respiration through the skin, although it has not yet been demonstrated by exact methods that the skin is really absorbing small quantities of oxygen; and it is even now not yet beyond doubt whether the small amount of carbonic acid may not be ascribed, perhaps, to the decomposition of the excretions from the skin, and of the epidermis that is shed, as stated by Prof. Bunge.[233] Footnote 233: Professor Bunge: Lehrbuch der Physiologie des Menschen, Leipzig, 1905. But if it is not possible to prove the certainty of respiration through the skin to a degree that would satisfy the postulates of exact science, on the other hand the observations of ancient savants, whose chemical knowledge and powers of observation were not inferior to those of their present successors, in spite of a deficient education in chemistry, all point to its existence, and we shall have to take it into account. As a rule the various products of skin perspiration cannot be seen, as they are eliminated in the form of vapor. This evaporation takes place in this invisible form, however, only when it is not checked or when it is not produced in excessive quantity. But when such evaporation is hindered by clothing that is impermeable to such products, such as rubber clothes or fur, or when it is too abundant, then it will be eliminated in the form of water through the pores, and will appear in drops. The human body loses from 1 to 1½ liters of this moisture, that is to say, sweat, in a day. Such checking of evaporation produces a very disagreeable feeling, a fact that indicates the existence of skin respiration; as does also the circumstance that persons clothed in a way that permits of the circulation of air to the skin and the elimination of the products of perspiration, and who also use other means for maintaining a good hygiene of the skin, such as a bath, are always in better health than those who neglect these points.