354 CHLORINE. 



but that water is not basic, for on dissolving the salt again, it is 

 found still to be a metaphosphate. But let this hydrated 

 metaphosphate be heated to 300, and without losing anything, 

 it changes completely, and becomes a pyrophosphate, the water 

 which was constitutional before, being now basic. The formulae 

 of the salt in its two states, exhibits to the eye the nature of 

 the internal change which has occurred in it : 



1. Hydrated metaphosphate of soda . NaO, PO 5 + HO, 

 2. Pyrophosphate of soda and water . NaO, HO + PO 5 . 



In describing the three classes of phosphates, with their rela- 

 tions to each other, I have been thus minute, partly because 

 considerable explanatory detail was required, from the extent of 

 the subject, but principally that we might avail ourselves of 

 the light which the phosphates have thrown upon the constitu- 

 tion of the class of organic acids, and upon the function of water 

 in many compounds. Indeed, phosphoric acid is one of the 

 links by which mineral and organic compounds are connected. 

 And it may be reasonably supposed that it is that pliancy of 

 constitution, which we have studied, that peculiarly adapts 

 this, above all other mineral acids, to the wants of the animal 

 economy. 



SECTION X. 



CHLORINE. 

 Eq. 442.65 or 35.47 ; Cl ; density 2440; ["T~|. 



This body was discovered by Scheele in 1774, but was believed 

 to be of a compound nature, till Gay-Lussac and Thenard in 

 1809, shewed that it might reasonably be considered a simple 

 substance. It is to the powerful advocacy of Davy, however, 

 who entered upon the investigation shortly afterwards, that the 

 establishment of the elementary character of chlorine is princi- 

 pally due, and to him it is indebted for the name it now bears, 

 which is derived from x^ w i oy ? yellowish green, and refers to 

 its colour as a gas, elementary bodies being generally named 

 from some remarkable quality or important circumstance in 

 their history. Chlorine is the leading member of a well-marked 

 natural family, to which also bromine, iodine and fluorine belong. 

 Phosphorus, carbon, hydrogen, sulphur, and most of the pre- 



