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stances, and of those that produce most heatin combustion, will pass through a metallictissue that will interrupt the flame of lessinflammable substances, or those that producelittle heat in combustion. Or the tissue being"the same, and impermeable to all flames atcommon temperatures, the flames of themost combustible substances, and of thoseAvhich produce most heat, will most readilypass through it when it is heated, and eachwill pass through it at a different degree oftemperature. In short, all the circum-stances which apply to the effect of coolingmixtures upon flame, will apply to coolingperforated surfaces. Thus, the flame ofphosphuretted hydrogene at common tem-peratures, will pass through a tissue suffi-ciently large not to be immediately choakedup by the phosphoric acid formed, and thephosphorus deposited.* A tissue of 100apertures to the square inch, made of wireof one-sixtieth, will at common temperatures

* If a tissue containing above 700 apertures to thesquare inch be held over the flame of phosphorus orphosphuretted hvdrogene, it does not transmit the flametill it is sufficiently heated to enable the phosphorus topass through it in vapour. Phosphuretted hydrogene isdecomposed in flame, and acts exactly like phosphorus.