The book of lantern ; being a practical guide to the working of the optical (1888)

THE BOOK OF THE LANTERN". 45 mixture of these two gases is a strictly mechanical, not a chemical one. What we mean is this. If it were possible by any means to make visible and magnify the particles of air, we should be able to distinguish the atoms of oxygen and of nitrogen side by side, but in the proportion of one to four. It might be compared to a mixture of pepper and salt, which, although it looks gray to the unaided sight, would, under the microscope, show plainly the independent grains of both constitutents. (It is curious to note that a chemical mixture of the two gases, in which their atoms combine to form a new compound, produces that useful anaesthetic, nitrous oxide—laughing gas.) It has long been the dream of chemists that oxygen might be produced direct from the atmosphere by separating its atoms from the atoms of nitrogen with which it is associated but not combined. Indeed, a plan by which this could be accom- plished has long been known, but it happens to be one of those numerous method which in theory are perfect, but which when reduced to practice are found to be encumbered by various difficulties. But as a new industry is founded upon the process referred to, and its success has been assured by a patient conquest of the numerous practical difficulties associated with it, we cannot do better than describe it. " It was long ago demonstrated by Boussingault that when the substance called baryta, otherwise the oxide of barium, was heated to a low redness, it would absorb oxygen from air submitted to it. He further showed that if this com- pound were then raised to a higher temperature, the oxygen thus absorbed would be given off once more, and the