curds when agitated with soap,
lathered with difficulty, and very imperfectly; but not the least
ebullition could be discovered upon dropping in spirit of sal ammoniac,
or solution of salt of tartar, though I had taken care to render the
latter free from causticity by impregnating it with fixed air.

EXPERIMENT V.

The distilled water saturated with fixed air neither effervesced, nor
shewed any clouds, when mixed with the fixed or volatile alkali.

EXPERIMENT VI.

No curd was formed by pouring this water upon an equal quantity of milk,
and boiling them together.

EXPERIMENT VII.

When agitated with soap, this water produced curds, and lathered with
some difficulty; but not so much as the distilled water mixed with
vitriolic acid in the very small proportion above-mentioned. The same
distilled water without any impregnation of fixed air lathered with soap
without the least previous curdling. River-water, and a pleasant
pump-water not remarkably hard, were compared with these. The former
produced curds before it lathered, but not quite in so great a quantity
as the distilled water impregnated with fixed air: the latter caused a
stronger curd than any of the others above-mentioned.

EXPERIMENT VIII.

Apprehending that the fixed air in the distilled water occasioned the
coagulation, or separation of the oily part of the soap, only by
destroying the causticity of the _lixivium_, and thereby rendering the
union less perfect betwixt that and the tallow, and not by the presence
of any acid; I impregnated a fresh quantity of the same distilled water
with fixed air, which had passed through half a yard of a wide
barometer-tube filled with salt of tartar; but this water caused the
same curdling with soap as the former had done, and appeared in every
respect to be exactly the same.

EXPERIMENT IX.

Distilled water saturated with fixed air formed a white cloud and
precipitation, upon being mixed with a solution of _saccharum saturni_.
I found likewise, that fixed air, after passing through the tube filled
with alkaline salt, upon being let into a phial containing a solution of
the metalic salt in distilled water, caused a perfect separation of the
lead, in the form of a white powder; for the water, after this
precipitation, shewed no cloudiness upon a fresh mixture of the
substances which had before rendered it opaque.

NUMBER II.

_A Letter from Mr. HEY to Dr. PRIESTLEY, concerning the Effects
of fixed