CHEMISTRY. 



41 



Hypothesis 

 of'Stahl. 





Elements was by Dr Hockc. According to him, there is an 

 f ingredient in air capable of dissolving combustibles 

 Chemittry. w j,en their temperature is sufficiently raised. The 

 > "^ Y ^^ solution takes place with such rapidity, that it occa- 

 sions light and heat, which, in his opinion, weremere 

 notions. The quantity of this solvent in air is not 

 great. Hence the reason why so great a proportion 

 of air is necessary to support combustion. This hy- 

 pothesis was embraced by Mayow, but without ma- 

 king any great addition either to its evidence or pro- 

 bability. 



Bcccher and Stahl soon after advanced another, 

 which was much more universally embraced. Ac- 

 cording to them, all combustible substances contain 

 in them a certain body called phlogiston, to which 

 they owe their combustibility. This substance is 

 the same in all combustible bodies. They owe their 

 diversity to other ingredients combined with the 

 phlogiston. During the combustion, the phlogiston 

 separates, and the incombustible ingredients remain 

 behind. The light and the heat are occasioned by 

 the violent motion into which the phlogiston is thrown 

 during its emission. 



Light being considered as a body, occasioned a 

 change in the Stahlian theory. Phlogiston was con- 

 sidered as nothing else than light fixed in bodies. 

 When heat, in consequence chiefly of the discoveries 

 of Dr Black, came to be considered as a body, the 

 opinion respecting phlogiston got a new modification. 

 It was considered as a subtile fluid, the same with the 

 ether of Hooke and Newton, which occasioned gra- 

 vity, and gave the bodies, called heat and light, the 

 peculiar motions which produce in us the sensations 

 of heat and light. 



Dr Priertley first attempted to account for the ne- 

 cessity of air for combustion. Air, according to him, 

 has an affinity for phlogiston ; it draws it out of the 

 combustible body, and combines with it. But if so, 

 whence come the heat and the light, which make their 

 appearance in all cases of combustion ? According to 

 Dr Crawford, they existed in the air, and were dis- 

 placed by the phlogiston when it united with that 

 fluid. 



These modifications of the Stahlian theory were 

 evidently improvements. But they left the nature of 

 phlogiston altogether out of view. Kirwan first at- 

 tempted to ascettain what this substance was, and to 

 prove it the same with what is now called hydrogen 

 gat. This opinion he supported in an ingenious Es- 

 say on Phlogiston ; and it was embraced by many of 

 the most respectable chemists in Europe. 



Meanwhile, Mr Lavoisier had been investigating 

 the subject with the minutest attention, and, after a 

 very long, elaborate, and ingenious examination, had 

 satisfied himself, that, in every case of combustion, 

 oxygen unites with the burning body. For a long 

 time nobody would accede to his opinion ; but at 

 last, in 1785, Bi-rthollet and Fourcroy joined hirr, 

 and soon after Guyton-Morveau came over to his 

 sentiments. They wrote a refutation of Mr Kirwan's 

 essay, which was so satisfactory, that Mr Kirwan 

 himself came over to their opinion. And after a short, 

 but pr tty violent controversy, the Lavoisierian theory 

 of combustion was universally adopted. According 

 to this 'heory, combustion consists of two processes, 



VOL. VI. PART I. 



Elernrm 

 of 



Of Priest- 

 ley. 



Cf I.avoi- 

 utr. 



a cnnibinatlon and a decomposition. The oxygen of 

 the air combines with the combustible, and gives out 

 the heat and light with which it was previously vni- 

 ted. '""" 



The following observations may, perhaps, contri- 

 bute somewhat to elucidate what is still obscure in 

 this curious process. 



All bodies, as far as combustion is concerned, may 

 be divided into sitpjMrlcrs, combustibles, and incom- 

 bnstibles. By supporters, are meant certain bodies, 

 not indeed capable of burning, but combustion can- 

 not go on without their presence. Air, for example, 

 is a supporter. Combustibles and incombusliblcs re- 

 quire no explanation. 



Oxygen and chlorine are the only simple support- 

 ers known. When they combine with an incombus- 

 tible, they form a compound supporter. The fol- List of sup. 

 lowing are all the supporters at present known : porters. 



1. Oxygen. 



2. Chlorine. 



3. Air. 



4. Nitrous oxide. 



5. Nitrous gas. 



6. Nitric acid. 



7. Euchloric gas. 



The combustibles are cither the simple substances 

 which have been already described, or combinations 

 of these with each other, or with oxygen without 

 combustion ; in which last case, they may be called 

 combustible oxides. 



During combustion, the oxygen or chlorine of the Products, 

 supporter always combines with the combustible, and 

 forms with it a new substance, which may be called 

 a product of combustion. Now every product is ei- 

 ther, 1. Water; 2. An acid; or, 3. A metallic 

 oxide. 



Some products are capable of combining with an Partial 

 additional dose of oxygen. But this combination is 

 never attended with combustion, and the product, in 

 consequence, is converted into a supporter. Such 

 compounds may be called partial supporters, as a part 

 only of the oxygen which they contain is capable of 

 supporting combustion. 



Since oxygen is capable of supporting combustion 

 only in the supporters and partial supporters, it is 

 clear that it is in a different state in these bodies from, 

 what it is in products. It is probable that, in sup- 

 porters, it contains combined with it a considerable 

 quantity of heat, which is wanting in products. 



It is probable, that combustible bodies contain 

 light as a constiiueiit. For the quantity of light tion ex. 

 emitted during combustion depends upon the com- plamd 

 bustible ; while the heat seems, in some measure at 

 least, to depend upon the oxygen. If these two 

 suppositions be admitted, the phenomena of combus- 

 tion admit of an easy explanation. The base of the 

 oxygen and of the combustible combine together, and 

 form the product ; while the heat of the one and the 

 light of the other in Irke manner unite, and fly off in 

 the form of fire. 



3. Percussion. 



It is well known, 'that h-at is produced by the per- PercuwicJi. 

 cussion of hard bodies Against each other. Iron may 



