CHEMICAL SCIENCE. 197 



passed through the porous tube, instead of the hydrogen in the former 

 experiment, while carbonic acid gas traverses the space between the 

 two tubes. The gases that emerge are collected over a bath containing 

 potash, to absorb the carbonic acid, and received in small glass jars. 

 When the furnace is in activity, the tubes yield an explosive mixture 

 of oxygen and hydrogen the elements of water. 



Thus it appears that part of the water is decomposed or " dissociated " 

 in the porous tube. " The hydrogen, attracted (appele) by the carbonic 

 acid in the annular interspace, has traversed the walls of the porous 

 tube, and separated itself by the simple action of a filter from the oxy- 

 gen which remains in the interior tube. A considerable quantity of 

 carbonic acid is attracted in a contrary direction, according to the rule 

 established in the preceding experiments, and mingles with the oxy- 

 gen." This is the broad explanation which M. Deville gives, but he 

 remarks that the action is in reality more complicated ; as, when the 

 hydrogen comes in contact with the heated carbonic acid, some car- 

 bonic oxide is formed, and a certain quantity of the latter gas is found 

 to replace the hydrogen. It is also difficult to avoid the escape of some 

 hydrogen, which leaves the oxygen in excess, and the water contains 

 enough air to effect the result. " The carbonic acid .determines the 

 separation of the gases by endosmose ; but it may also act mechani- 

 cally." 



In explanation of these facts, M. Deville states, that the temperature 

 of the combustion of hydrogen in oxygen is not equal to 2500 C., at 

 which point the volume of the gases, estimated at C., is multiplied 

 tenfold, and beyond which the complete decomposition of water takes 

 place. " But this decomposition is accompanied by a considerable ab- 

 sorption of latent heat, to the extent required to keep the molecules of 

 oxygen and hydrogen at a distance beyond the radius of the sphere of 

 their affinity. Thus the decomposition of a body resembles the ebulli- 

 tion of a liquid, the principal characteristic of which is invariableness 

 of temperature under the same pressure." Admitting the comparison 

 between decomposition and ebullition, M. Deville regards " dissociation," 

 or partial decomposition at a temperature below the decomposing point, 

 as resembling the evaporation of liquids below their boiling point. 

 " If," he observes, " you shut up some water in a small vessel, at ordi- 

 nary temperature, the evaporation is slight, on account of the tension 

 that is produced when vapor is formed ; but, if you introduce a piece of 

 chloride of calcium, the water evaporates until that substance is satu- 

 rated, the tension remaining constant all the time." He imagines the 

 carbonic acid to carry off the dissociated gases just as the chloride of 

 calcium absorbs the vapor, and then the process of dissociation, like 

 that of evaporation, is enabled to go on. 



Porosity of Platinum. In prosecuting further experiments in re- 

 gard to the above subject, some interesting facts relative to the porosity 

 of platinum and its endosmotic action were elicited. They caused a 

 platinum tube to be drawn out of one piece, so as to be free from all 

 solder, and to present a uniform and unbroken surface. This platinum 

 tube was introduced into a porcelain one, so that an empty cylindrical 

 space was left all round between the two, properly stopped at each 

 end. Through this space, a constant current of hydrogen was made to 

 pass, by means of two glass tubes inserted at the extremities, so as not 



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