232

general remarks.

nity with the red muriate, or sulphate of iron, produced by thecombination of oxigen, is gradually evolved, and mingled withthe residual nitrogen gas.

The impregnated solution with green muriate is more rapid inits operation than the solution with green sulphate. In cases whenthese salts cannot be obtained in a state of absolute purity, thecommon sulphate of iron of commerce may be employed. Onecubic inch of moderately strong impregnated solution, is capableof absorbing five or six cubic inches of oxigen, in common pro-cesses ; but the same quantity must never be employed for morethan one experiment.

GENERAL REMARKS.

In all these different methods of analysing air, it is necessaryto operate on air of a determinate density, and to take care thatthe residuum be neither more condensed nor dilated than the airwas when first operated on. If these things are not attended to,no depcndance whatever can be placed upon the result of theexperiments, how carefully soever they' may have been perform-ed. It is therefore necessary to place the air before and afterthe examination into water of the same temperature. If thisand several other little circumstances have been attended to, forinstance a change in the height of the barometer, &c. wefind that air is composed of 0.22 of oxigen gas, and 0.“8 ofnitrogen gas by bulk. But as the weight of these two gasesis not exactly the same, the proportion of the component partsby weight will differ a little ; for as the specific gravity of oxigengas is to that of nitrogen gas as 135: 115, it follows that 100parts of air are composed ly weight of about74 nitrogen gas26 oxigen gas

100

The air of this metropolis examined by means of Davy's eudio-meter, was found in all the different seasons of the year to con-