Such substances as caustic soda, lime, bromide of magnesium, &c., are capable of absorbing carbonic acid, and have been used, but modern submarine vessels rely on compressed-air compartments. M. Calmette, in his account of his voyage under water in the _Morse_, says that thanks to the labours of a commission formed by M. de Lanessan the difficulty of respiration has been satisfactorily overcome, with the result that the crew can remain for sixteen hours under water without the slightest strain. This may refer to the discovery by M. Georges Jaubert of a chemical substance of comparatively light weight which in one single operation can not only completely remove from vitiated air the carbonic acid, water vapour, and other non-respirable products, but can also automatically restore to it in exchange the exact mathematical quantity of oxygen which it lacks. In other words the substance when placed in contact with air vitiated by respiration can completely regenerate it and restore to it its original qualities. This wonderful substance, the composition of which has not yet been made public, has been the subject of some communications by Dr. Laborde to the Paris Academy of Sciences. In these it is mentioned that experiments were being made under the auspices of the Minister of Marine, and that these had proved that with 3 to 4 kilos of this new product it was possible for a man to live for twenty-four hours in an hermetically closed chamber. VISION WHEN SUBMERGED. When completely submerged the submarine boat is practically blind, and it is impossible to steer it by direct vision through the water. However slow its course the steersman would be unable to stop it before an obstacle which rose suddenly into the restricted circle of his aquatic vision, and he is therefore obliged to steer his course by means of bearings taken before descending. At one time inventors believed that some light would come down through the water to help the steersman, but it is now acknowledged that once below the surface the boat is in impenetrable darkness. Some have proposed the use of a powerful electric projector which would emit a beam of light sufficient to light up a path 50 to 60 metres long in front of the submarine. Unfortunately such an arrangement is not possible in practice. In the first place, such a projector would be of great size and weight and would require a considerable amount of current, but even if it were installed on a submarine it would be of no use to those on board for the reason that they would be placed immediately behind it and would be able to distinguish nothing owing to the great glare. The projector’s light would also be likely to betray the position of the submarine owing to the rays finding their way to the surface.