I want you to understand that this property of gravitation is never lost, that every substance possesses it, that there is never any change in the quantity of it; and, first of all, I will take as illustration a piece of marble. Now this marble has weight--as you will see if I put it in these scales; it weighs the balance down, and if I take it off, the balance goes back again and resumes its equilibrium. I can decompose this marble and change it, in the same manner as I can change ice into water and water into steam. I can convert a part of it into _its own_ steam easily, and shew you that this steam from the marble has the property of remaining in the same place at common temperatures, which _water_-steam has not. If I add a little liquid to the marble, and decompose it[6], I get that which you see--[the Lecturer here put several lumps of marble into a glass jar, and poured water and then acid over them; the carbonic acid immediately commenced to escape with considerable effervescence]--the appearance of boiling, which is only the separation of one part of the marble from another. Now this [marble] steam, and that [water] steam, and all other steams _gravitate_, just like any other substance does--they all are attracted the one towards the other, and all fall towards the earth; and what I want you to see is, that _this_ steam gravitates. I have here (fig. 2) a large vessel placed upon a balance, and the moment I pour this steam into it, you see that the steam gravitates. Just watch the index, and see whether it tilts over or not. [The Lecturer here poured the carbonic acid out of the glass in which it was being generated into the vessel suspended on the balance, when the gravitation of the carbonic acid was at once apparent.] Look how it is going down. How pretty that is! I poured nothing in but the invisible steam, or vapour, or gas which came from the marble, but you see that part of the marble, although it has taken the shape of air, still gravitates as it did before. Now, will it weigh down that bit of paper? [Placing a piece of paper in the opposite scale.] Yes, more than that; it nearly weighs down this bit of paper. [Placing another piece of paper in.] And thus you see that _other_ forms of matter besides solids and liquids tend to fall to the earth; and, therefore, you will accept from me the fact--that _all_ things gravitate, whatever may be their form or condition. Now _here_ is another chemical test which is very readily applied. [Some of the carbonic acid was poured from one vessel into another, and its presence in the latter shewn by introducing into it a lighted taper, which was immediately extinguished.] You see from this result also that it gravitates. All these experiments shew you that, tried by the balance, tried by pouring like water from one vessel to another, this steam, or vapour, or gas, is, like all other things, attracted to the earth. [Illustration: Fig. 3. and Fig. 4.]