86 Mr. Daniell on the Barometer. 



point. He accordingly did me the favour to extract the following 

 particulars : — 



In June, 1823, he made a 'mixture of one volume of oxygen 

 and two volumes of hydrogen ; with this he filled five dry bottles 

 over mercury, and also four bottles over water. He left the 

 glasses inverted over mercury and water, placing three mercury 

 and two water bottles in the windows, so as to receive the sun's 

 rays and day-light ; and two mercury and two water bottles he 

 placed in a dark place. In July, 1824, he examined the bottles; 

 the water bottle in the light contained hydrogen and some 

 common air, and there was no alteration of volume ; the 

 mercury bottle in light contained common air only. The water 

 bottles in the dark place showed no alteration of volume, and 

 the air contained in them proved to be the original mixture ; the 

 mercury bottles in the same situation contained nothing but com- 

 mon air. 



Now, if I had not known, from the authority of the President, 

 that the Council of the Royal Society did not think this sufficient 

 proof, I should very confidently have concluded from these facts, 

 that a fluid which has attraction enough for glass, to enable it to 

 wet4ts surface, effectually prevents the passage of gases into or 

 out of vessels of that substance ; while a fluid which does not 

 wet the surface permits their slow penetration. I should, more- 

 over, have ventured to affirm, that the case of the confined air 

 is exactly analogous to that of the barometer ; for its escape and 

 the admission of the atmospheric air can only be in virtue of the 

 law discovered by Mr. Dalton, that the gases are as vacua to one 

 another. The inference is also pretty strong, that the filtration 

 takes place along the surface of the glass, and not through the 

 pores of the fluid. 



It has been attempted, I understand, to contravert this conclu- 

 sion, by the observation that gases have been preserved a consi- 

 derable time by mercury ; but when it is considered that the 

 slightest film of moisture, or any foulness of the mercury, will 

 form a connexion between the metal and the glass, the objec- 

 tion can be of no avail, unless these circumstances have been at- 

 tended to. To ensure the maximum of the effect which I have 



