ON SPONTANEOUS INFLAMMATIONS. 219' 



On changing the experiment, and rubbing a cylinder ofan^ wlthacy- 

 one of thefe woods l^elween two tablets of the other, for ex- two^tab^ets."** 

 ample, a cylinder of poplar between two tablets of mulberry, 

 the augmentation of the rubbed furfaces which are in conta6l 

 with the air, produced a much more confiderable heat, and 

 nearly all the wpods mentioned above took fire. 



The effe6t of the fri61ion alfo varies according as the woods Thcdireaion of 

 employed, particularly if they are of the fame fpecies, ^r e ^^oo A^fkal ^t 

 rubbed in the direction of the grain of the wood, or when thefriftion. 

 grains of the woods crofs each other. In the nrft cafe the 

 fridion and the heat are much more confiderable than in the 

 fecond. 



In large machines, in which there is a great deal of friction. Preventives 

 the heating is prevented by directing a continual current of ^^^'^^'^jj^^^'j^^ _ 

 cold water on the rubbing furfaces: in common machines, tion by friftion. 

 and in coaches, waggons, 8fc. it is diminilhed by covering the 

 rubbing furfaces with fome oily matter. There have been 

 many examples, during the great heat of fummer, of coaches, 

 and other machines (ubjefled to a rapid motion, having taken 

 fire, becaufe the greafing them had been negleded. The 

 greafe, by hardening on the rubbing furfaces, inftead of di- 

 minithing the friftion, increafes it; and as this covering is very 

 combuftible, it renders fpontaneous inflammation flill more 

 eafy. It is alfo preferable, in many circumftances, to rub 

 machines with foap, talc, plumbago, or other fubftances, 

 which, without being oily, are very unduous to the touch. 



2. ABion of the Sun. 



By concentrating the folar rays with convex glafTes or con- Aftionof the 

 cave mirrors, the ftrongeft heat is produced, all forts of*^""* 

 combuftibles are fet on fire, and the moft refracloiy fubftances 

 are melted : it may happen that other bodies may be found in 

 circumftances, in which, without our concurrence, they pro- 

 duce the efFeds of glafTes and burning mirrors: allhough thefe 

 effeds are rather phyfical than chemical, it is neverthelefs 

 eflential to make them known, to guard againft their danger. 

 There are examples of- fires produced by large glafs bottles, 

 filled with-water and expofed to the fun, in an apartment. 

 Whenever the form of the veflel is nearly limilar to that of a 

 '■ lenticular 



