90 DISCOVERY OF IODINE. 



coming converted into the chloride of phosphorus ; the potas- 

 sium also combining with the gas with intense energy, and 

 becoming the chloride of that metal. The strict analogy 

 of the formation and nature of this compound with those of 

 the chloride of sodium, or common salt, naturally led to the 

 explanation of the chemical constitution of that substance; 

 and it was thus shown that Chlorine, the subject of the 

 Lecture, enters in a large proportion (36 parts out of 60 

 by weight) into the composition of a substance which is 

 very important in the preparation of food, and for many other 

 purposes of common life. The action of chlorine upon quick- 

 lime was then noticed, and the chloride of lime, or Bleaching 

 Powder, resulting from that action, exhibited to the classes ; 

 and the property of a solution of chlorine of destroying vege- 

 table colours being also stated, and shown by experiment, the 

 use of the cholride of lime, in the operation of Bleaching, was 

 explained. Notices of the effects of chlorine in destroying con- 

 tagion, and of its liquefaction by pressure, completed the sub- 

 ject*. 



" The next of the simple supporters of combustion which 

 we have to consider, was discovered by M. Courtois, of Paris, 

 a manufacturer of chemical articles, in 1812. It was stated in 

 the preceding lecture, that the three bodies, which, in addition 

 to oxygen, form this class, viz. chlorine, iodine, and bromine, 

 occur in nature associated together in sea-water, in the form of 

 acids united with the alkali soda, constituting various salts. 

 Now the various kinds of sea-wrack, or sea-weed, such as are 

 called Fuel by naturalists, and indeed almost all the plants 

 which naturally grow on the sea-shore, have the power of de- 

 composing the sea-water, and of taking into their own sub- 

 stance some of the chemical bodies which it contains. Thus 

 they retain, among them, the soda, and when they are burnt, 

 the ashes which are left contain a considerable quantity of that 

 alkali. On this account these plants are burnt in large quan- 



* Four compounds of chlorine having thus been noticed, their composi- 

 tion was shown in a diagram by making the quantity of chlorine constant, 

 .36, and then stating the respective quantities of the other bodies uniting 

 with it to form the chlorides ; thus : 



Chlorine. Base. 

 Chloride of Phosphorus.. 36 12 



Potassium . . 36 40 



Sodium 36 24 



Lime 36 28 



It was preferred to state the composition in these numbers, in order to 

 familiarize the pupils to them, as the representative or equivalent numbers, 

 or the atomic weights, of the bodies named j preparatory to the specific in- 

 struction to be given on that subject. 



