88 DIFFERENCE BETWEEN MIXTURE AND COMBINATION. 



composition, by synthesis by mixing together these two gases, 

 in the proper proportions, and ascertaining whether the mix- 

 ture possessed the properties of atmospheric air. Accordingly, 

 four measures of Azote and one of Oxygen being mixed, (as a 

 near approximation to the actual proportions of 79 and 21) it 

 was shown that a match, a taper, and inflamed phosphorus, 

 immersed in it, neither burnt with increased splendour, as in 

 Oxygen, nor were extinguished, as in Azote, but burnt just 

 as they did in the air of the room. 



In explaining the constitution of atmospheric air as a mere 

 mixture of Oxygen and Azote, it became necessary to explain 

 the difference between mechanical mixture and chemical com- 

 bination. In mixtures, it was stated, that the properties are 

 intermediate to those of the two elements, a statement accu- 

 rately justified by the properties of atmospheric air, compared 

 with those respectively possessed by Oxygen arid Azotic gases; 

 but that a chemical combination possesses properties alto- 

 gether different from those of its elements, and acts upon other 

 bodies in a manner peculiar to itself, not possessing the inter- 

 mediate properties of its elements, but new ones, which are not 

 exhibited by either of them. As examples, the five chemical 

 combinations of Azote and Oxygen were referred-to, and the 

 properties of one of them, the Deutoxide of Azote, or Nitrous 

 Gas, a chemical combination of 1 4? Azote and 1 6 Oxygen, by 

 weight, taken in particular ; its properties, not possessed by 

 either of its elements, of extinguishing most burning bodies, 

 but supporting the combustion of phosphorus, and of produ- 

 cing red fumes on mixing with the air, being experimentally 

 shown. Red Lead, Sulphurous acid gas, Sulphuret of Iron, 

 and Sulphate of Magnesia, were also named and exhibited, 

 as examples of chemical combination. 



Hitherto, the process of combustion, in every instance in 

 which it had been exhibited in these Lectures, had taken place 

 either in Oxygen gas, or in some mixture or combination con- 

 taining it. " But there are," (besides Oxygen) it was now ob- 

 served, " three other bodies, which, from the powerful manner 

 in which they promote intense chemical action, attended by 

 the evolution, or giving out, of heat and light, have been 

 termed Supporters of Combustion ,- a term, however, which is 

 one rather of convenience, than of absolute scientific truth, as 

 I shall explain, in the course of our chemical inquiries. 



" The other supporters of combustion, then, are the sub- 

 stances named Chlorine, Iodine, and Bromine, which are 

 intimately related among themselves ; especially by their all 

 being coloured bodies, while Oxygen, as we have seen, has no 

 colour, and is invisible ; and they are all likewise found asso- 



