590 FOURTH UEPOKT — 1831'. 



burst out into open ignition. It is to be remarked, how- 

 ever, that this low combustion differs from ordinary inflamma- 

 tion in its products, and that the transition from one to the 

 other is not gradual, but abrupt, and attended with a slight 

 explosion. 



Several of the metals exhibit the phaenomenon of low com- 

 bustion. The action is limited in most cases by the speedy 

 formation of a coat of oxide on the surface of the metal ; but in 

 the case of arsenic, whose oxide is volatile, a pale flame sur- 

 rounds it at any temperature capable of raising it into vapour, 

 and continues until the metal is consumed. The fresh filings 

 of zinc, iron, cobalt, antimony, tungsten, and copper, become 

 momentarily luminous when thrown on an iron heated below 

 redness. Potassium presents the phsenomena of low combus- 

 tion at ordinary temperatures ; in fact, the rapid tarnishing of 

 its surface, after it is cut or rubbed, is accompanied by the 

 evolution of a faint hght, which becomes brighter if the tempe- 

 rature is raised. 



The light of this low combustion is worthy of notice, which 

 varies somewhat in different shades of pallid or bluish white. 

 It becomes a question, what constitutes this light ? That of ordi- 

 nary flame is supposed to consist of minute particles of the com- 

 bustible, or of its product, in a state of incandescence. Sir H. 

 Davy extended this supposition even to the low combustion of 

 phosphorus, attributing its feebleness of heat to the extreme 

 paucity and tenuity of the particles of phosphoric acid thus 

 raised to a white heat. The author conceives that if this were 

 the cause, there would be a red tinge occasionally present, as 

 the result of the cooling of these particles to the red degree of 

 heat. In most ordinary flames such a red tinge occurs, and is 

 particularly apparent in daylight, but the author has never 

 seen it in low combustion. The lowest luminous degree of 

 heat has commonly been stated to be red, that called by arti- 

 sans cherry red being the first visible degree. If, however, 

 we examine the phsenomena of incandescence in a room other- 

 wise perfectly dark, by watching a large piece of iron cool 

 from a red heat, we shall find that, before it ceases to be lumi- 

 nous, it loses wholly its red light, and appears of a pale or 

 milky white. This, although fainter, is precisely the colour of 

 the lights of low combustion. 



The author then drew a brief comparison between the pro- 

 ducts of low combustion, and those of fermentation and putri- 

 faction, in which he noticed a new process for the expeditious 

 manufacture of vinegar. In this case an infusion of malt, or 

 sweet-wort, is made to drop through a room full of faggots of 



