_[g CONDITION OF EQUILIBRIUM. 



I was led by necessity to choose between spongy platina and the binoxide of nitrogen 

 After an experience of some extent in the employment of this gas, it has not appeared 

 to deceive me ; it is, indeed, an eligible method in gaseous analysis where oxygen is con- 

 cerned. The mode of manipulation is as follows : with the sliding rod eudiometer, throw 

 100 measures of the gas under trial above the surface of water that has been duly ex- 

 posed to the atmosphere, and contained in an inverted bell, rather wide in proportion to 

 its depth; one made of the belly of a glass retort or a cupping-glass answers very well. 

 Then add 100 measures of the binoxide of nitrogen if the gas is suspected to be poor 

 in oxygen, but 200 or more if the gas is richer, always observing to have the binoxide 

 in excess. After the lapse of a minute the absorption is complete ; measure the resi- 

 due, and one fourth of the diminution gives the volume of oxygen : this method is anal- 

 ogous to that of Gay Lussac. Some idea of its correctness may be formed from the cir- 

 cumstance that, of 73 analyses of the air, the mean result of the amount of oxygen is 

 20-58 per cent. My measuring rod divides each volume into decimals by a vernier ar- 

 rangement ; but for most purposes of analysis this is unnecessary. 



50. It results, from the observations which have been made on caoutchouc by Dr. 

 Mitchell, that oxygen passes through it with much more facility than nitrogen. Atmo- 

 spheric air is also reputed to be a mixture, and not a chemical compound ; it was there- 

 fore an object to try whether pure oxygen might not be obtained by forcing air through 

 such a membrane, filtering, or, in fact, straining it through a gum elastic bag. A thin 

 piece of this substance was therefore tied tightly over a tube an inch in diameter and 

 six inches long; the tube was then filled with mercury in such a manner that the great 

 weight might not burst the caoutchouc ; it was then inverted and exposed to the atmo- 

 sphere. The membrane bulged into the tube in a deep hemispherical form; in about 

 an hour its under surface was studded with bubbles of gas, and in the course of time 

 several cubic inches passed. This, on analysis, by means of binoxide of nitrogen, was 

 found not to differ sensibly from atmospheric air. A similar result was also obtained 

 when a thin serous membrane, a piece of peritoneum stripped from the liver, was sub- 

 stituted for the gum elastic. No indications whatever could be obtained that atmospheric 

 air was decomposed during the process. Nor is it difficult to understand and explain 

 how this happens, when a foreign force, equivalent to a pressure of six inches of mer- 

 cury, is brought to bear so advantageously on the action of a very thin membrane; for 

 in the case of the gum elastic, the thickness could not be estimated at more than ^th 

 of an inch, and the serous membrane was so porous that it could not sustain so heavy 

 a pressure without immediate leakage ; the united gas, whatever it may be, is at once 

 forced through, the barrier being unable to stop it. A case of the same kind is met 

 with when porous charcoal is used: pressure forces a gas through it entirely unchanged; 

 but if the effects of that pressure be avoided, chemical decompositions of a decisive 

 character may ensue, as we shall shortly have occasion to see. To obtain these chem- 

 ical effects, it is necessary that the barrier should not only have no pores of sensible 

 size, but that no adventitious or foreign forces be brought to act on the passing gas ; in 

 proportion as these conditions are fulfilled, the success of the experiment is more perfect ; 

 and thus, as we shall proceed to point out, it is possible to strain the nitrogen out of 

 atmospheric air, and procure b'y that means oxygen of greater or less purity. 



