As to the dust-producing capacity of

From Chambers' Journal.

PLANTS.

the different substances experimented EFFECTS OF FROSTS AND THAWS UPON upon, common salt was found to be one of the most active. When burned in a fire or in alcohol flame, it gave an intensely fog-producing atmosphere. But salt, again, was quite outdone by sulphur which was the most active substance experimented upon. It gave rise to a fog so dense that it was impossible to see through a thickness of two inches of it.

.

The dust particles which form the nuclei of fog and cloud, must not be confounded with the minute dust-motes which are revealed to us by a beam of sunlight when shining into a darkened room; because these dust-motes can be entirely removed by heat, and yet the air remain active as a cloud-producer. The heat would seem to break up the larger motes which reflect the light, into smaller and invisible ones. When dust-particles are spoken of, therefore, it must be understood that reference is not made to such motes as the sunlight reveals, and which are comparatively large, but to those infinitesimally small particles which are quite invisible.

SOME observations were made at Giessen last winter by Herr Hoffmann which throw light on the way in which plants are injured in time of hard frost. It is well known that plants and trees situated in the bottom of a valley suffer much more from cold and frost than those in a higher situation. This is due to the fact that the valley, if surrounded by hills and high grounds, not only retains its own cold of radiation, but also serves as a reservoir for the cold heavy air which pours down into it from the neighboring heights. It is thus that the higher grounds in Switzerland are warmer than the valleys or gorges, as in these the cold collects as in so many basins. It is also found in this country that plants and shrubs which survive the severity of winter on ground raised above the level of the valley, perish where grown in the valley itself. The great advantage of a hilly position is thus apparent, and has been amply proved by Herr Hoffmann's observations at Giessen. Here he found that the plants so situated took little or no This theory of Mr. Aitken's is not un-harm from the intense cold; while quite likely to lead to some discussion in its relation to the question of city fogs, such as those that darken and defile London. Deductions, however, must not be allowed to weigh against facts which are ascertainable by experiment; and although, in the larger field of nature outside the laboratory of the physicist, agencies may be called into play to modify in some respects the conclusions based upon these experiments, yet, looking at the matter as a simple discovery in the domain of meteorological science, the facts ascertained by Mr. Aitken in this connection are of immense value. Among other things, they prove the beneficial service of cotton-wool respirators to persons who suffer from asthmatic or pulmonary affections, or even to healthy persons who reside in districts liable to be invaded by fogs or mists. As we have seen, it was impossible to raise any fog in a receiver containing air which had been filtered through cotton-wool, the air being absolutely pure, and uncontaminated by those microscopic particles which not unfrequently contain in them the germs of disease and death. The discovery of the connection between fogs and dust seems to us to be one of the most useful, as it is one of the most curious, scientific achievements of the century.

near, in the valley, there was extensive injury. The injury, too, decreased in proportion to elevation above the valley. As to the immediate effect of temperature upon plants, the author is of opinion that it is not a particular degree of cold that kills a plant, but the amount of quick thawing. This was illustrated in one case by the curious fact that one and the same bush -a species of box-was killed in its foliage on the south side, while on the north the foliage remained green. The sudden change of temperature produced by quick thawing, was considered to be some degrees less for the plants in a high situation and for the shady sides of the half-killed shrubs. The higher situations are in this respect also favorable to plantlife; because, while the frost is not so severe as in the valley, the effect of thawing winds is found to be the same for both. The plants on the higher grounds are therefore subjected to less strain by sudden variations from a low to a high temperature, and the reverse, than their congeners in the valleys.

These facts are of importance in determining questions as to the sites of country-houses and gardens, and the more or less hardy character of the plants and shrubs most likely in the particular situation to survive the frosts of winter.

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