During summer, when the weather is sultry, and the sky assumes that beautiful blue tinge so entirely its own, dew is formed in the greatest abundance, owing to the phenomena which are requisite for its deposition being then most favourably combined. It was long supposed by naturalists that this precipitation depended on the cooling of the atmosphere towards evening, when the solar rays began to decline; but it was not properly understood until M. Prevost published his theory of the radiation of caloric (which has since been generally adopted), which was as follows:--“That all bodies radiate caloric constantly, whether the objects that surround them be of the same temperature of themselves, or not.” According to this view, the temperature of a body falls whenever it radiates more caloric than it absorbs, and rises whenever it receives more than it radiates; which law serves to produce an equality of temperature. Such is exactly the case as regards the earth: during the day it receives a supply of heat from the sun’s rays, and as it is an excellent radiator of caloric, as soon as the shades of evening begin to fall, the earth imparts a portion of its caloric to the air, and the atmosphere having no means of imparting its caloric in turn, except by contact with the earth’s surface, the stratum nearest the earth becomes cooled, and consequently loses the property of holding so much moisture in the state of vapour, which becomes deposited in small globular drops. The stratum of air in immediate contact with the earth having thus precipitated its moisture, becomes specifically lighter than that immediately above it, which consequently rushes down and supplies its place; and in this manner the process is carried on until some physical cause puts a stop to it either partly or wholly. It is well known that dew is deposited sparingly, or not at all, in cloudy weather, the clouds preventing free radiation, which is so essential for its formation; that good radiators, as grass, leaves of plants, and filamentous substances in general, reduce their temperature in favourable states of the weather to an extent of ten or fifteen degrees below the circumambient air; and whilst these substances are completely drenched with dew, others that are bad radiators, such as rocks, polished metal, sand, &c., are scarcely moistened. From the above remarks it will appear evident that dew is formed most abundantly in hot climates, and during summer in our own, which tends to renovate the vegetable kingdom by producing all the salubrious effects of rain without any of its injurious consequences, when all nature seems to languish under the scorching influence of a meridian sun. Hoarfrost is formed when the temperature becomes so low as 32 degrees Fahrenheit; the dew being then frozen on falling, sometimes assuming very fantastic forms on the boughs and leaves of trees, &c., which sparkle in the sunshine like so many gems of purest ray.