TRANSACTIONS OF THE SKCTIONS. 581 



are at least two hydrates of peroxide of iron : the muriate of 

 that which contains least water is red in solution, and the muri- 

 ate of the other, yellow; but these muriates pass readily into 

 each other. Mr. R. Phillips observed of the red muriate, 

 that it is precipitated by an access of acid, which, it may be re- 

 marked, establishes an analogy between it and the muriate of 

 the nitric acid peroxide of tin, which possesses the same pro- 

 perty. 



Metallic peroxides can in general be obtained by the appli- 

 cation of a moderate heat to their hydrates, in a state in which 

 they are the debris of hydrates, and not neat chemical com- 

 pounds. Upon heating peroxides in this condition to redness, 

 they generally glow or become spontaneously incandescent at a 

 particular temperature, (a phaenomenon to which the attention 

 of chemists has been particularly directed by Berzelius,) and 

 lose their solubility in acids at the same time. Till they have 

 undergone this change, they are not absolute or proper perox- 

 ides. Various salts, such as phosphates, antimoniates, &c., 

 exhibit the same phaenomenon when heated ; but they all had 

 possessed water, which is essential to their first constitution, 

 but not to their second. 



The doctrine of isomerism, or that two bodies may exist of 

 the same composition, but differing in properties, has been pro- 

 posed by Berzelius as a sequence from such facts as the pre- 

 ceding. But the propriety of the inference may be doubted. 

 Most, if not all cases of apparent isomerism may be explained 

 by reference to one or other of the following facts : 



1. Water is essential to the constitution of many bodies. 

 Thus, what have been called metaphosphoric acid, pyrophos- 

 phoric acid, and common phosphoric acid, are three different 

 phosphates of water, or compounds of one absolute phosphoric 

 acid with three different proportions of water. 



2. A particular condition of bodies must be recognised, in 

 which they are the debris of some compound, and not proper 

 chemical compounds of their constituents. Thus, on heating 

 a certain borate of water and magnesia to redness, water only 

 is expelled ; but what remains is not a simple borate of magnesia, 

 but a mixture of boracic acid and magnesia, from which the 

 former may be dissolved out by water. Stucco in a. state for 

 setting is in this particular condition. But this is a depart- 

 ment of corpuscular philosophy which stands much in want of 

 further development. 



3. The proximate constitution of many bodies may be widely 

 different, of which the ultimate composition is the same. Thus 

 the cyanic acid of Wohler is undoubtedly an oxide of cyanogen, 

 but we have no evidence that cyanogen exists in fulminic acid, 



