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[ 48 ] only arise from a difference in the cause that acts upon them.

95. We may form our ideas of the manner in which the invisible humor distributes itself; from that in which h all kinds of visible vapours are dif- fused We see them seperate re-unite, fly off from certain places, rush into others, and in short yield to every impression of the air. The motion pecu- liar to their own particles, which I look upon as the cause of their elasticity*, is not sufficiently ra- pid, and the vapours themselves arc too thick to overcome always the contrary motion of the air. This, I believe, is what contributes the chief difference between vapours, and the igneous Auid, as far as relates to the power of putting themsevles into a state of equilibrium in the air, which is moving. The curren t of air carried towards a chimney which has fire in it, frees the room from smoke, and is but a very sight impediment to the diffusion of the heat through it. 96. Though the invisible vapours by reason of their excessive thinness are more capable of being put in equilibrium in the air than the visible ones, they are very far from having this property in as great degree as the heat Which leads me to think, that part of the difference observed between my hygrometers, even before sunrise, may have been owing to the unequal distribution of the hu≠ mor, though the two instruments were only at the distance of a foot from each other, without the in≠ terposition oi any solid body.

• The system I adopted on that point may be found in my

work upon the Modifications of the atmosphere. 97∑ I