Water, when taken up by the atmosphere, is not in an aqueous state, but is converted into vapor by the efficiency of heat, and then combines with more than five times the quantity of caloric than is required to bring ice-cold water to a boiling heat, and occupies a space 800 times greater than it does when in the form of water. A large portion of the matter of heat combining chemically with water, renders it specifically _lighter_; which is the cause of its rising and mixing with the atmosphere. The waters on the face of the earth would be dissipated in vapor by a small degree of heat, if we had no atmosphere. Under the pressure of the atmosphere water boils at 212°, but in vacuo it boils when heated only to 67°. On the contrary, if additional pressure be given to water by a Papin’s digester, it may be heated to 400°, without producing ebullition. However long we boil a fluid, in an open vessel, we cannot make it in the smallest degree hotter than the boiling point.[93] When arrived at this point, the vapor absorbs the heat, and carries it off as fast as it is generated. When water is received into the atmosphere, if the air be warm, it becomes so far changed by its union with the matter of heat as to be perfectly invisible. In this state it occupies a space 1,400 times greater than its ordinary liquid state. After vapor has remained some time in the atmosphere, it becomes in a measure condensed; and the particles of water of which it is composed unite, and form hollow vesicles, which accumulate together and produce clouds. How this is effected, those who have attentively considered the subject are not agreed. Dr. Thomson, after well investigating the matter, concludes, from all the facts, that “the formation of clouds and rain cannot be accounted for by a single principle with which we are acquainted.” It is, however, says Mr. Parkes, probable that _electricity_ alone is the primary cause. Saussure conjectures that it is the electrical fluid which surrounds these vesicles, and prevents them from dissolving in the air. And the idea of the formation of clouds by the agency of electricity was mentioned by Volta, and also by Dr. Franklin. [It is allowed by all, that clouds are formed by the aqueous vapors which are held suspended, or in solution, by the atmosphere. It is not a settled question, whether the air holds these vapors in solution, or merely suspended; and thus, keeping the particles asunder, prevents their condensation. This aqueous vapor is _invisible_ when perfectly in union with the air. When it begins to separate from the air, it becomes visible by condensation, in the form of _clouds_, _mists_, and _fogs_. When it is perfectly separated and sufficiently condensed it becomes _rain_, and when the temperature is sufficiently low to freeze the condensed drops, they become _snow_, or _hail_.