When a given quantity of air, saturated with water-vapor, is suddenly rarefied, a part of this vapor (it is known) is precipated as cloud, in consequence of the fall of temperature. In a paper recently contributed to the Journal de _Pharmaeie et de Chemic, M. Conlier states that to render this phenomenon more apparent he procured a large zinc tube 3 meters in length, and closed the ends of it with glass discs. He introduced a little water, compressed the air present, for a little, and then opened a cock at the side, so relieving the compression. Thereupon appeared a pretty thick cloud, through which the outline of a candle-flame could not be perceived. Having, after some days, however, repeated the experiment, he found that it did not always succeed; and he set himself to find the reason of this.
With this view, he obtained a three-necked flask, the bottom of which was covered with water. Two of the necks were furnished with cocks, while the third was connected by a caoutchouc tube with a balloon of caoutchouc, the compression of which, when both cocks were closed, produced an instant compression of the air in the flask ; on removing the pressure, the air expanded again, and a cloud was formed in the flask. The cloud spherules thus produced were of various sizes, and were in constant motion. On looking at a flame through the cloud, one observed the well-known colored rings.
Hitherto the formation of these clouds has been explained by precipitation of water in consequence of cooling. The following experiment shows, however, that this explanation is not exact. If the flask is allowed to stand some time undisturbed, the phenomenon is not obtained. Under circumstances that are apparently exactly the same as those of the experiment at first, the air remains perfectly clear. To explain this fact it must be supposed that this air has been altered in composition, and has lost a constituent, which gave it the property of becoming troubled in expansion. This element seems to be soluble in water; for if you vigorously shake the flask when it contains the cloud-giving air, the air becomes inoperative.
Carbonic acid, oxygen, and various other gases, are without influence on the phenomenon. In the water at the bottom of the flask no new constituent could be discovered. If it be wished to repeat