75. I believe, nevertheless, that I have observed many times, both in frogs and in animals with warm and red blood, such as cats and Guinea pigs, that the infiltration of oxygen gas into the cellular tissue gives, after a certain time, a brighter colour to the blood than this fluid presents in the artificial emphysemas which may be produced by carbonic acid gas, hydrogen gas, or by atmospheric air, in which circumstances the blood differs very little in colour from its natural shade. But in other cases oxygen gas has had no influence over the colour of the blood; so that, notwithstanding the many experiments that have been made on this point, I cannot state any general result. It appears, that the tonic powers of the cellular tissue, and of the coats of the vessels which ramify in it, receive a very varied influence from the contact of the gases, and that, according to the nature of that influence, the fibres contracting and becoming more or less firm render these parts more or less permeable, both to the æriform fluids, which have a tendency to escape from the blood to unite with that of the emphysema, and to this last fluid, if it tends to combine with the blood. This will doubtless explain the variations that I have observed. 76. Do the mucous surfaces exhale? The analogy of the skin would seem to lead to the belief of it; for it appears well proved, that the perspiration is not a transudation by the inorganic pores of the cutaneous surface, but a true transmission by vessels of a particular nature, and continuous with the arterial system. 77. It appears, at first, that the pulmonary perspiration which takes place on the surface of the bronchi, which has such connection with that of the skin, which increases or diminishes according to the decrease or augmentation of the other, and of which the composition is apparently of the same nature--it appears, I say, that the pulmonary perspiration is produced, at least in part, by the system of exhalent vessels; and that if the combination of the oxygen of the air concurs with the hydrogen of the blood to produce it, during the act of respiration, it is but in a very small quantity, and for that portion only which is purely aqueous. It is necessary to observe further on this subject, that the dissolution of the mucous fluid, which lubricates the bronchi, in the air that is constantly inspired and expired, furnishes a considerable portion of that vapour which rises from the lungs, and which is insensible in summer, but very apparent in winter.