132 Analysis of Scientific Books. 



Dr. Thomson lays down his own scheme of combustion, with 

 abundant technology. " All bodies in nature, "| says he, " as 

 far as combustion is concerned, may be divided into three 

 classes ; namely, supporters, combustibles, and incombustibles. 

 By supporters, I mean substances which are not themselves, 

 strictly speaking-, capable of undergoing combustion ; but their 

 presence is absolutely necessary in order that this process may 

 take place. Combustibles and incombustibles require no defini- 

 tion. The simple supporters, at present known, are three in 

 number ; namely, oxygen, chlorine, iodine. The compounds 

 which these three bodies make with each other, and with azote, 

 are likewise supporters." " During combustion, the supporter 

 (supposing it simple, or if compound, the oxygen, chlorine, or 

 iodine, excluding the base,) always unites with the combustible, 

 and forms with it a new substance, which I shall call a product 

 of combustion. Hence the reason of the change which com- 

 bustibles undergo by combustion. Now every product is either, 

 1. an acid ; 2. an oxide; 3. a chloride ; or, 4. an iodide. As' 

 light and heat are always emitted during combustion, but never 

 when a supporter combines with a combustible without com- 

 bustion, it is natural to suppose, that the supporters contain 

 either the one or the other of these bodies, or both of them. I 

 am disposed to believe that the supporters contain heat, while 

 that body in other cases is wanting, or at least not present in 

 sufficient quantity. My reason for this opinion is, that the heat 

 which is evolved during combustion, is always greatest when 

 the quantity of supporter which combines with the burn- 

 ing body is greatest ; but this is by no means the case with 

 regard to light." 



We have quoted the passage at some length, because it 

 exhibits the characteristic vice of our author, who is always mis- 

 taking scholastic subdivisions for inductive generalisation. 

 That man must take a narrow view of chemical phaenomena, who 

 ventures to construct the above crude distinctions ; for theory 

 it cannot be called. Does he not know that protoxide of chlo- 

 rine, as it comes in contact with iodine, both in the cold, pro- 

 duces combustion ? Here we have his supporters, strictly speak- 

 ing, of themselves and by themselves, capable of undergoing 

 combustion. Again, potassium burns in sulphuretted hydrogen ; 

 but where is his supporter, " whose presence is absolutely neces- 

 sary, in order that this process (combustion) may take place ?" 

 Potassium burns likewise in cyanogen, in the absence of all his 

 three supporters. What more powerful instances of combustion 

 can we adduce, than are exhibited in the explosion of the chlo- 

 ride and iodide of azote ? Yet none of Dr. Thomson's combusti- 

 bles is present. We deny that every product of combustion is 

 either an acid, an oxide, a chloride, or an iodide. The forma- 

 tion of the sulphurets, even in vacuo, is accompanied with vivid 



