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 



