bxpefmentsfor ifivesttgatingy &c. ' i^5 



(Zoning the azot it contains. If we tbink there has b^ch a 

 change, it is, in fact, because a little azol escapes at the 

 commencement of the absorption of the hydrogen or oxy- 

 giin 'f but shake the water and the gaf?es together, the whole 

 azot which was previously interposed iA thcJ' water will en- 

 ter into it and interpose itself as formerly, independently of 

 the hydrogen or oxygen gas, which it has beside absorbed. 



7. The preceding result is so coi'rect that we may exactly 

 analyse the atmospheric air by the simple absorbent action of 

 water. For this purpose it is sufficient that the water has 

 been previously impregnated with azot } it :then absorbs 

 precisely ^1 hundredths of the atmospheric air put in con- 

 tact with it, as a sulphuret would do. M. Marty asserts 

 that the water thus empioyed in great quantities, in order 

 that the operation may be more expeditious, is an excellent 

 eudiometer, and that he used it as such frequently. If we 

 have no aZot at hand, we may impregnate the water wil^ 

 this gas by shaking it with atmospheric air, and leaving it for 

 some time in contact with this air : by this means it absorbs 

 all the azot it ought to contain ; and the oxygen it takes at 

 the same time does not hinder it, according to the first ex- 

 periment, from absorbing in time, that of the air we mean 

 to analyse. M. Marty also uses this absorbent property of 

 water to ascertain if a given oxygen gas does or does not 

 contain azot ; for if it does contain it, the water saturated 

 with azot does not absorb it completely. 



XLII. Experiments for invest] gating the Cause of the co- 

 loured concentric RingSy discovered hj Sir Isaac Newton, 

 tetween two Object-glasses laid upon one another. jSy 

 WiLLiAM Herschel, LLD, F.R.S, 



[Concluded from p. 128.] 



XXV. Of the Action of the second Surface, 



Ag fings are fofrficd whe-n two glasses are laid upon- each 

 other, it is but reasonable to expect that the two- surfaced at 

 leasf which aref lacfidtogab^ fliouldhaiv^ an iitime<jRate^ effect 



N S upon 



