He first used ammonia, putting one drop into six cubic inches of water in a flask, and sunning this for one minute; the result was a considerable quantity of condensation, even with such a weak solution. When the flask was exposed for five minutes, the condensation by the action of the sunshine was made more dense. Hydrogen peroxide was tested in the same way, and it was found to be a powerful generator of nuclei. Curious is it that sulphurous acid is puzzling to the experimentalist for cloud formation. It gives rise to condensation in the dark; but sunshine very conclusively increases the condensation. Chlorine causes condensation to take place without supersaturation; sulphuretted hydrogen (which one always associates with the smell of rotten eggs) gives dense condensation after being exposed to sunshine. Though the most of these nuclei, due to the action of sunshine in the gases, remain active for cloudy condensation for a comparatively short space of time--fifteen minutes to half-an-hour--yet the experiments show that it is possible for the cloudy condensation to take place in certain circumstances in the absence of dust. This seems paradoxical in the light of the former beautiful experiments; but, in ordinary circumstances, dust is needed for the formation of clouds. However, supposing there is any part of the upper air free from dust, it is now found possible, when any of these gases experimented on be present, for the sun to convert them into nuclei of condensation, and permit of clouds being formed in dustless air, miles above the surface of the earth. In the lower atmosphere there are always plenty of dust-particles to form cloudy condensation, whether the sun shines or not. These are produced by the waste from the millions of meteors that daily fall into the air. But in the higher atmosphere, clouds can be formed by the action of the sun's rays on certain gases. This is a great boon to us on the earth; for it assures us of clouds being ever existing to defend us from the sun's extra-powerful rays, even when our atmosphere is fairly clear. This is surely of some meteorological importance. CHAPTER X DECAY OF CLOUDS From the earliest ages clouds have attracted the attention of observers. Varied are their forms and colours, yet in our atmosphere there is one law in their formation. Cloud-particles are formed by the condensation of water-vapour on the dust-particles invisibly floating in the atmosphere, up to thousands--and even millions--in the cubic inch of air. But observers have not directed their attention so much to the decay of clouds--in fact, the subject is quite new. And yet how suggestive is the subject!