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quantity and velocity of fire separately, but even its absoluteeffects on bodies, are unknown. The point to which abod) entirely deprived of fire can be condensed is equallyundetermined with the first instant of its expansion. Thecondensation of mercury, produced by a certain quan-tity of sal ammoniac and snow mixed together, is, in theconstruction of thermometers, the lowest point that weknow; while that, from experiments made in more northernclimates, it is clear that mercury can be much more con-densed. In the construction of pyrometers, the point of thegreatest condensation depends on the variable temperatureof the atmosphere. Thus these two instruments serve onlyto shew, by their graduation, the difference of expansion,according to the degree of fire that they contain or arepenetrated with.

8. The activity of fire considered as actually existing ina body, (6. N p 2.) depends not only on the quantity andvelocity of the ignited matter, but also on the mass of thebody in which it exists. On attempting to reduce, two fluidsof unequal density to the fame temperature, the most denserequires the greatest degree of heat: thus, when the activityand degree of fire which the two bodies contain are pro-portional to their respective masses, they are equally hot.On touching two bodies of unequal density, reduced to thefame temperature, the most dense appears the hottest; be-cause the hand being in contact with a greater number ofparticles in the denser body, the sensation excited is propor-tionally stronger.

9. The manner and law (6. N 0 ^.) under which firepenetrates bodies, is modified by the degree of its activity,the time of application, and the superficies, quality, ana mass,of the surrounding bodies ; for should any one of thesecircumstances vary, a different modification will result. Putinto the same fire two pieces of iron of the same quality,but of different sizes, the smaller piece will become hotmuch sooner than the larger one; and if the i'urface of thelarger piece be increased by flattening it with a hammer,it will become hot in less time. This diversity of effectsmay be equally remarked on comparing a piece of iron witha stone, or any other body, put into the fame fire.

These observations on the facility with which fire insi-nuates itself into bodies, are equally applicable to the facility

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